diff --git a/apps/hq/src/api.ts b/apps/hq/src/api.ts
index 485630a..e611e37 100644
--- a/apps/hq/src/api.ts
+++ b/apps/hq/src/api.ts
@@ -59,6 +59,7 @@ import {
 } from './repos-aws'
 import { pullMonthlyCosts } from './aws-costs'
 import { clientProfitability, duesAging, moduleRevenue, type DateRange } from './repos-reports'
+import { listUsageRateCards, setUsageRateCard } from './repos-rate-card'
 import { commitImport, stageCsv, verificationReport } from './import-apex'
 import { documentHtml, documentHtmlSample } from './templates'
 import { renderPdf } from './pdf'
@@ -376,6 +377,30 @@ export function apiRouter(
       res.status(400).json({ ok: false, error: err instanceof Error ? err.message : String(err) })
     }
   })
+  // D23 quantity-tiered rate card (SMS packs and future usage-billed modules).
+  r.get('/modules/:id/rate-card', requireAuth, async (req, res) => {
+    const id = String(req.params['id'] ?? '')
+    if ((await getModule(db, id)) === null) {
+      res.status(404).json({ ok: false, error: 'Module not found' }); return
+    }
+    res.json({ ok: true, rateCards: await listUsageRateCards(db, id) })
+  })
+  r.post('/modules/:id/rate-card', requireAuth, requireOwner, async (req, res) => {
+    const id = String(req.params['id'] ?? '')
+    if ((await getModule(db, id)) === null) {
+      res.status(404).json({ ok: false, error: 'Module not found' }); return
+    }
+    try {
+      const body = req.body as { effectiveFrom?: unknown; bands?: unknown }
+      if (typeof body.effectiveFrom !== 'string' || !Array.isArray(body.bands)) {
+        throw new Error('Provide effectiveFrom (YYYY-MM-DD) and bands [{ minQty, ratePaise }]')
+      }
+      await setUsageRateCard(db, staffId(res), id, body.effectiveFrom, body.bands as { minQty: number; ratePaise: number }[])
+      res.json({ ok: true, rateCards: await listUsageRateCards(db, id) })
+    } catch (err) {
+      res.status(400).json({ ok: false, error: err instanceof Error ? err.message : String(err) })
+    }
+  })
   r.post('/modules/:id/prices', requireAuth, requireOwner, async (req, res) => {
     const id = String(req.params['id'] ?? '')
     if ((await getModule(db, id)) === null) {
diff --git a/apps/hq/src/db.ts b/apps/hq/src/db.ts
index f7503a7..6a7f140 100644
--- a/apps/hq/src/db.ts
+++ b/apps/hq/src/db.ts
@@ -105,6 +105,15 @@ CREATE TABLE IF NOT EXISTS module_price_book (
   id TEXT PRIMARY KEY, module_id TEXT NOT NULL, edition TEXT NOT NULL DEFAULT 'standard',
   kind TEXT NOT NULL, price_paise INTEGER NOT NULL, effective_from TEXT NOT NULL
 );
+-- D23 quantity-tiered pricing (SMS packs and any future usage-billed module). A dated
+-- "rate card" = all bands sharing (module_id, effective_from). A purchase of N units
+-- resolves to the band with the highest min_qty <= N; N below the lowest min_qty is
+-- under the minimum order. Rates are integer paise per unit; a new card is a new dated
+-- set of rows, never an edit (config over code — old invoices keep their old rate).
+CREATE TABLE IF NOT EXISTS usage_rate_band (
+  id TEXT PRIMARY KEY, module_id TEXT NOT NULL, effective_from TEXT NOT NULL,
+  min_qty INTEGER NOT NULL, rate_paise INTEGER NOT NULL
+);
 CREATE TABLE IF NOT EXISTS client_module (
   id TEXT PRIMARY KEY, client_id TEXT NOT NULL, module_id TEXT NOT NULL,
   status TEXT NOT NULL DEFAULT 'quoted' CHECK (status IN
diff --git a/apps/hq/src/migrations-pg.ts b/apps/hq/src/migrations-pg.ts
index 3dfa859..6e893f5 100644
--- a/apps/hq/src/migrations-pg.ts
+++ b/apps/hq/src/migrations-pg.ts
@@ -215,6 +215,15 @@ CREATE TABLE IF NOT EXISTS ticket (
 ALTER TABLE module ADD COLUMN IF NOT EXISTS field_spec text NOT NULL DEFAULT '[]';
 ALTER TABLE client_module ADD COLUMN IF NOT EXISTS field_values text NOT NULL DEFAULT '{}';
 ALTER TABLE client_module ADD COLUMN IF NOT EXISTS secrets_enc text;
+`,
+  },
+  {
+    id: '004-usage-rate-band',
+    sql: `
+CREATE TABLE IF NOT EXISTS usage_rate_band (
+  id text PRIMARY KEY, module_id text NOT NULL, effective_from text NOT NULL,
+  min_qty bigint NOT NULL, rate_paise bigint NOT NULL
+);
 `,
   },
 ]
diff --git a/apps/hq/src/repos-documents.ts b/apps/hq/src/repos-documents.ts
index 22e4692..98f28a8 100644
--- a/apps/hq/src/repos-documents.ts
+++ b/apps/hq/src/repos-documents.ts
@@ -4,6 +4,7 @@ import { writeAudit } from './audit'
 import type { DB } from './db'
 import { getClient } from './repos-clients'
 import { getModule, priceOn, type Kind } from './repos-modules'
+import { resolveUsageRate } from './repos-rate-card'
 import { dismissQuoteFollowups, getNumberSetting } from './repos-reminders'
 import { nextDocNo } from './series'
 
@@ -179,8 +180,24 @@ async function buildLines(db: DB, inputs: DraftLineInput[], onDate: string, warn
     const mod = await getModule(db, line.moduleId)
     if (mod === null) throw new Error(`Module not found: ${line.moduleId}`)
     const edition = line.edition ?? 'standard'
-    let unitPricePaise = line.unitPricePaise
-      ?? await priceOn(db, line.moduleId, line.kind, edition, onDate)
+    // Price resolution order: (1) an explicit per-line override always wins (special deal);
+    // (2) a quantity-tiered rate card if the module has one (D23 — SMS packs price by count);
+    // (3) the dated price book. A tiered module below its minimum order blocks the save.
+    let unitPricePaise = line.unitPricePaise ?? null
+    if (unitPricePaise === null) {
+      const tiered = await resolveUsageRate(db, line.moduleId, line.qty, onDate)
+      if (tiered === null) {
+        unitPricePaise = await priceOn(db, line.moduleId, line.kind, edition, onDate)
+      } else if (tiered.kind === 'below_min') {
+        if (warnings === undefined) {
+          throw new Error(`${mod.code}: minimum order is ${tiered.minQty.toLocaleString('en-IN')} units — enter at least that many`)
+        }
+        warnings.push(`${mod.code}: below the ${tiered.minQty.toLocaleString('en-IN')} minimum order`)
+        unitPricePaise = 0
+      } else {
+        unitPricePaise = tiered.ratePaise
+      }
+    }
     if (unitPricePaise === null) {
       // Strict (no warnings sink): refuse — same message save has always thrown.
       if (warnings === undefined) {
diff --git a/apps/hq/src/repos-rate-card.ts b/apps/hq/src/repos-rate-card.ts
new file mode 100644
index 0000000..923cee6
--- /dev/null
+++ b/apps/hq/src/repos-rate-card.ts
@@ -0,0 +1,88 @@
+import { uuidv7 } from '@sims/domain'
+import { writeAudit } from './audit'
+import type { DB } from './db'
+
+/**
+ * Quantity-tiered pricing (D23) — a dated per-module "rate card" of volume bands.
+ * SMS packs are the first user: 50k→₹0.40, 100k→₹0.37, 200k→₹0.34, 300k→₹0.30 per SMS,
+ * with 50,000 the minimum order. Generic on purpose — any usage-billed module can have
+ * one. Rates are integer paise per unit; a new card is a new dated set of rows (config
+ * over code), so issued invoices keep the rate they were billed at.
+ */
+
+export interface RateBand { minQty: number; ratePaise: number }
+
+export type UsageRateResult =
+  | { kind: 'rate'; ratePaise: number }
+  | { kind: 'below_min'; minQty: number }
+  | null // no rate card for this module/date — caller falls back to the price book
+
+/**
+ * Resolve the per-unit rate for `qty` units of a module on `onDate`. Picks the latest
+ * dated card, then the band with the highest `min_qty <= qty`. Returns null when the
+ * module has no card (not usage-tiered); `below_min` when qty is under the minimum order.
+ */
+export async function resolveUsageRate(
+  db: DB, moduleId: string, qty: number, onDate: string,
+): Promise<UsageRateResult> {
+  const card = await db.get<{ effective_from: string }>(
+    `SELECT effective_from FROM usage_rate_band
+     WHERE module_id=? AND effective_from<=? ORDER BY effective_from DESC LIMIT 1`,
+    moduleId, onDate,
+  )
+  if (card === undefined) return null
+  const bands = await db.all<{ min_qty: number; rate_paise: number }>(
+    `SELECT min_qty, rate_paise FROM usage_rate_band
+     WHERE module_id=? AND effective_from=? ORDER BY min_qty`,
+    moduleId, card.effective_from,
+  )
+  const minQty = bands[0]!.min_qty
+  if (qty < minQty) return { kind: 'below_min', minQty }
+  let chosen = bands[0]!
+  for (const b of bands) if (b.min_qty <= qty) chosen = b
+  return { kind: 'rate', ratePaise: chosen.rate_paise }
+}
+
+/** Replace (or create) the dated card for (module, effectiveFrom). Owner-gated at the route. */
+export async function setUsageRateCard(
+  db: DB, userId: string, moduleId: string, effectiveFrom: string, bands: RateBand[],
+): Promise<void> {
+  if (bands.length === 0) throw new Error('A rate card needs at least one band')
+  if (!/^\d{4}-\d{2}-\d{2}$/.test(effectiveFrom)) throw new Error('effectiveFrom must be YYYY-MM-DD')
+  const sorted = [...bands].sort((a, b) => a.minQty - b.minQty)
+  const seen = new Set<number>()
+  for (const b of sorted) {
+    if (!Number.isInteger(b.minQty) || b.minQty <= 0) throw new Error('min_qty must be a positive integer')
+    if (seen.has(b.minQty)) throw new Error(`duplicate band min_qty: ${b.minQty}`)
+    seen.add(b.minQty)
+    if (!Number.isInteger(b.ratePaise) || b.ratePaise < 0) throw new Error('rate_paise must be a non-negative integer (paise)')
+  }
+  await db.transaction(async () => {
+    await db.run(`DELETE FROM usage_rate_band WHERE module_id=? AND effective_from=?`, moduleId, effectiveFrom)
+    for (const b of sorted) {
+      await db.run(
+        `INSERT INTO usage_rate_band (id, module_id, effective_from, min_qty, rate_paise) VALUES (?, ?, ?, ?, ?)`,
+        uuidv7(), moduleId, effectiveFrom, b.minQty, b.ratePaise,
+      )
+    }
+    await writeAudit(db, userId, 'set_rate_card', 'module', moduleId, undefined, { effectiveFrom, bands: sorted })
+  })
+}
+
+export interface RateCard { effectiveFrom: string; bands: RateBand[] }
+
+/** All dated cards for a module, newest first. */
+export async function listUsageRateCards(db: DB, moduleId: string): Promise<RateCard[]> {
+  const rows = await db.all<{ effective_from: string; min_qty: number; rate_paise: number }>(
+    `SELECT effective_from, min_qty, rate_paise FROM usage_rate_band
+     WHERE module_id=? ORDER BY effective_from DESC, min_qty`,
+    moduleId,
+  )
+  const byDate = new Map<string, RateBand[]>()
+  for (const r of rows) {
+    const list = byDate.get(r.effective_from) ?? []
+    list.push({ minQty: r.min_qty, ratePaise: r.rate_paise })
+    byDate.set(r.effective_from, list)
+  }
+  return [...byDate.entries()].map(([effectiveFrom, bands]) => ({ effectiveFrom, bands }))
+}
diff --git a/apps/hq/test/sms-rate-card.test.ts b/apps/hq/test/sms-rate-card.test.ts
new file mode 100644
index 0000000..dcb92c4
--- /dev/null
+++ b/apps/hq/test/sms-rate-card.test.ts
@@ -0,0 +1,105 @@
+// apps/hq/test/sms-rate-card.test.ts — D23: SMS volume-band pricing, money locked to the paisa.
+import { describe, it, expect } from 'vitest'
+import { openDb } from '../src/db'
+import { seedIfEmpty } from '../src/seed'
+import { createClient } from '../src/repos-clients'
+import { createModule } from '../src/repos-modules'
+import { resolveUsageRate, setUsageRateCard, listUsageRateCards } from '../src/repos-rate-card'
+import { createDraft, issueDocument, getDocument, prepareDraft } from '../src/repos-documents'
+
+// The founder's real card: ₹0.40 / 0.37 / 0.34 / 0.30 per SMS = 40/37/34/30 paise.
+const BANDS = [
+  { minQty: 50_000, ratePaise: 40 },
+  { minQty: 100_000, ratePaise: 37 },
+  { minQty: 200_000, ratePaise: 34 },
+  { minQty: 300_000, ratePaise: 30 },
+]
+
+async function world() {
+  const db = openDb(':memory:')
+  await seedIfEmpty(db) // company state 32, GST18
+  const c = await createClient(db, 'u1', { name: 'Karapuzha SCB', stateCode: '32' })
+  const sms = await createModule(db, 'u1', { code: 'SMS', name: 'Bulk SMS', allowedKinds: ['usage'] })
+  await setUsageRateCard(db, 'u1', sms.id, '2026-04-01', BANDS)
+  return { db, c, sms }
+}
+
+describe('SMS volume-band resolution (D23)', () => {
+  it('picks the band by count and enforces the minimum order', async () => {
+    const { db, sms } = await world()
+    const rate = async (n: number) => resolveUsageRate(db, sms.id, n, '2026-07-16')
+    expect(await rate(50_000)).toEqual({ kind: 'rate', ratePaise: 40 })
+    expect(await rate(75_000)).toEqual({ kind: 'rate', ratePaise: 40 })   // in [50k,100k)
+    expect(await rate(100_000)).toEqual({ kind: 'rate', ratePaise: 37 })
+    expect(await rate(150_000)).toEqual({ kind: 'rate', ratePaise: 37 })
+    expect(await rate(200_000)).toEqual({ kind: 'rate', ratePaise: 34 })
+    expect(await rate(300_000)).toEqual({ kind: 'rate', ratePaise: 30 })
+    expect(await rate(500_000)).toEqual({ kind: 'rate', ratePaise: 30 })  // >= top band
+    expect(await rate(49_999)).toEqual({ kind: 'below_min', minQty: 50_000 })
+    // A module with no card is not usage-tiered.
+    const other = await createModule(db, 'u1', { code: 'X', name: 'X' })
+    expect(await resolveUsageRate(db, other.id, 100_000, '2026-07-16')).toBeNull()
+  })
+
+  it('resolves by business date — an older card stays in force before the new one', async () => {
+    const { db, sms } = await world()
+    await setUsageRateCard(db, 'u1', sms.id, '2027-04-01', [{ minQty: 50_000, ratePaise: 45 }])
+    expect(await resolveUsageRate(db, sms.id, 50_000, '2026-12-31')).toEqual({ kind: 'rate', ratePaise: 40 })
+    expect(await resolveUsageRate(db, sms.id, 50_000, '2027-05-01')).toEqual({ kind: 'rate', ratePaise: 45 })
+  })
+})
+
+describe('SMS billing through the engine (money locked)', () => {
+  const cases = [
+    { count: 50_000, taxable: 20_000_00, total: 23_600_00 },
+    { count: 100_000, taxable: 37_000_00, total: 43_660_00 },
+    { count: 200_000, taxable: 68_000_00, total: 80_240_00 },
+    { count: 300_000, taxable: 90_000_00, total: 1_06_200_00 },
+  ]
+  for (const tc of cases) {
+    it(`${tc.count.toLocaleString('en-IN')} SMS → ₹${tc.taxable / 100} + 18% GST = ₹${tc.total / 100}`, async () => {
+      const { db, c, sms } = await world()
+      const draft = await createDraft(db, 'u1', {
+        docType: 'INVOICE', clientId: c.id,
+        lines: [{ moduleId: sms.id, qty: tc.count, kind: 'usage' }],
+      })
+      const inv = await issueDocument(db, 'u1', draft.id)
+      const full = (await getDocument(db, inv.id))!
+      expect(full.taxablePaise).toBe(tc.taxable)
+      expect(full.payablePaise).toBe(tc.total)
+      // Intra-state (client 32 = company 32): CGST + SGST split, IGST zero.
+      expect(full.cgstPaise + full.sgstPaise).toBe(tc.total - tc.taxable)
+      expect(full.igstPaise).toBe(0)
+    })
+  }
+
+  it('a save below the minimum order is refused; preview warns instead of throwing', async () => {
+    const { db, c, sms } = await world()
+    await expect(createDraft(db, 'u1', {
+      docType: 'INVOICE', clientId: c.id, lines: [{ moduleId: sms.id, qty: 10_000, kind: 'usage' }],
+    })).rejects.toThrow(/minimum order is 50,000/)
+    const prev = await prepareDraft(db, {
+      docType: 'INVOICE', clientId: c.id, lines: [{ moduleId: sms.id, qty: 10_000, kind: 'usage' }],
+    }, { permissive: true })
+    expect(prev.warnings.some((w) => /minimum/.test(w))).toBe(true)
+    expect(prev.totals.payablePaise).toBe(0)
+  })
+
+  it('an explicit per-line unit price overrides the card (special deal)', async () => {
+    const { db, c, sms } = await world()
+    const draft = await createDraft(db, 'u1', {
+      docType: 'INVOICE', clientId: c.id,
+      lines: [{ moduleId: sms.id, qty: 100_000, kind: 'usage', unitPricePaise: 35 }], // ₹0.35 special
+    })
+    const inv = await issueDocument(db, 'u1', draft.id)
+    expect((await getDocument(db, inv.id))!.taxablePaise).toBe(35_000_00) // 100k × 0.35, not the 0.37 band
+  })
+
+  it('lists dated cards newest first', async () => {
+    const { db, sms } = await world()
+    const cards = await listUsageRateCards(db, sms.id)
+    expect(cards).toHaveLength(1)
+    expect(cards[0]!.bands).toHaveLength(4)
+    expect(cards[0]!.effectiveFrom).toBe('2026-04-01')
+  })
+})