diff --git a/config/defaults/billing.pricing.constants.js b/config/defaults/billing.pricing.constants.js
new file mode 100644
index 000000000..bd7aa1336
--- /dev/null
+++ b/config/defaults/billing.pricing.constants.js
@@ -0,0 +1,32 @@
+/**
+ * Billing pricing constants — devkit-shipped export contract with safe defaults.
+ *
+ * Downstreams override the VALUES (not the contract) by setting `billing.pricing.*`
+ * in their `config/defaults/<project>.config.js`. The global `config` exposes the
+ * merged result at `config.billing.pricing`. Importers should always read through
+ * `config.billing.pricing.PLAN_QUOTAS` etc., NOT from this file directly — that
+ * way downstream values win the glob-merge.
+ *
+ * Why ship from devkit:
+ * - Every downstream running billing wants the same export shape.
+ * - Migrations + contract tests + costs service all benefit from a single import path.
+ * - Trawl had this file at `modules/billing/config/billing.pricing.constants.js` with
+ *   6+ importers — promoted upstream in plan `2026-06-02-trawl-billing-residual-cleanup.md`.
+ *
+ * @module billing.pricing.constants
+ */
+
+/** @type {string} YYYY.MM pricing version (e.g. '2026.05'). Default 0.0.0 = unset. */
+export const PRICING_VERSION = '0.0.0';
+
+/** @type {Record<string, number>} Weekly meter quota in compute units per plan. */
+export const PLAN_QUOTAS = { free: 0 };
+
+/** @type {Record<string, number>} Compute unit multipliers per feature key. */
+export const RATIOS = {};
+
+/** @type {Record<string, { monthly: number, annual: number }>} Stripe price cents per plan. */
+export const STRIPE_PRICE_CENTS = {};
+
+/** @type {Record<string, number>} Stripe price cents per extras pack. */
+export const STRIPE_PACK_CENTS = {};
diff --git a/config/defaults/development.config.js b/config/defaults/development.config.js
index 0e100b216..79676b493 100644
--- a/config/defaults/development.config.js
+++ b/config/defaults/development.config.js
@@ -1,3 +1,11 @@
+import {
+  PRICING_VERSION,
+  PLAN_QUOTAS,
+  RATIOS,
+  STRIPE_PRICE_CENTS,
+  STRIPE_PACK_CENTS,
+} from './billing.pricing.constants.js';
+
 const config = {
   app: {
     title: 'Devkit Node - Development Environment',
@@ -142,6 +150,16 @@ const config = {
       },
     },
   },
+  billing: {
+    /**
+     * Pricing constants contract — safe devkit defaults.
+     * Downstream projects override these values in their own config file.
+     * These values are also directly importable from
+     * `config/defaults/billing.pricing.constants.js` for migrations and
+     * standalone tooling that cannot import the full config object.
+     */
+    pricing: { PRICING_VERSION, PLAN_QUOTAS, RATIOS, STRIPE_PRICE_CENTS, STRIPE_PACK_CENTS },
+  },
 };
 
 export default config;
diff --git a/lib/services/tests/billing-pricing-constants.contract.unit.tests.js b/lib/services/tests/billing-pricing-constants.contract.unit.tests.js
new file mode 100644
index 000000000..0e75d2955
--- /dev/null
+++ b/lib/services/tests/billing-pricing-constants.contract.unit.tests.js
@@ -0,0 +1,26 @@
+import { describe, test, expect } from '@jest/globals';
+import {
+  PRICING_VERSION,
+  PLAN_QUOTAS,
+  RATIOS,
+  STRIPE_PRICE_CENTS,
+  STRIPE_PACK_CENTS,
+} from '../../../config/defaults/billing.pricing.constants.js';
+
+describe('billing.pricing.constants — devkit contract:', () => {
+  test('PRICING_VERSION is the safe default 0.0.0', () => {
+    expect(PRICING_VERSION).toBe('0.0.0');
+  });
+  test('PLAN_QUOTAS is the safe default { free: 0 }', () => {
+    expect(PLAN_QUOTAS).toEqual({ free: 0 });
+  });
+  test('RATIOS is the safe default empty object', () => {
+    expect(RATIOS).toEqual({});
+  });
+  test('STRIPE_PRICE_CENTS is the safe default empty object', () => {
+    expect(STRIPE_PRICE_CENTS).toEqual({});
+  });
+  test('STRIPE_PACK_CENTS is the safe default empty object', () => {
+    expect(STRIPE_PACK_CENTS).toEqual({});
+  });
+});
diff --git a/modules/billing/services/billing.refund.service.js b/modules/billing/services/billing.refund.service.js
new file mode 100644
index 000000000..2f71f00d3
--- /dev/null
+++ b/modules/billing/services/billing.refund.service.js
@@ -0,0 +1,53 @@
+/**
+ * Module dependencies
+ */
+import getStripe from '../lib/stripe.js';
+
+/**
+ * @function refundCharge
+ * @description Initiate a Stripe refund for a given charge.
+ *              Wraps stripe.refunds.create with a deterministic idempotency key so
+ *              retries on network failures never create duplicate refunds.
+ *              The actual ledger debit happens via the `charge.refunded` webhook
+ *              (single source of truth) — this service ONLY initiates the Stripe refund.
+ *
+ * @param {string} stripeChargeId - Stripe charge ID (ch_xxx). Must be non-empty.
+ * @param {number|undefined} [amountCents] - Amount to refund in cents. Omit for full refund. Must be > 0 if provided.
+ * @param {Object} [options={}] - Optional Stripe refund options.
+ * @param {string} [options.reason] - Optional Stripe refund reason.
+ * @returns {Promise<Object>} The Stripe refund object.
+ */
+// biome-ignore lint/correctness/useQwikValidLexicalScope: false positive — Node.js service, not Qwik
+const refundCharge = async (stripeChargeId, amountCents, options = {}) => {
+  if (options === null || typeof options !== 'object' || Array.isArray(options)) {
+    throw new Error('invalid argument: options must be a plain object');
+  }
+  const { reason } = options;
+  if (typeof stripeChargeId !== 'string' || stripeChargeId.trim() === '') {
+    throw new Error('invalid argument: stripeChargeId must be a non-empty string');
+  }
+  if (amountCents !== undefined) {
+    if (!Number.isInteger(amountCents) || amountCents <= 0) {
+      throw new Error('invalid argument: amountCents must be a positive integer');
+    }
+  }
+
+  const stripe = getStripe();
+  if (!stripe) throw new Error('Stripe is not configured');
+
+  const idempotencyKey = `refund_${stripeChargeId}_${amountCents ?? 'full'}`;
+
+  const params = {
+    charge: stripeChargeId,
+    reason: reason || 'requested_by_customer',
+  };
+  if (amountCents !== undefined) {
+    params.amount = amountCents;
+  }
+
+  return stripe.refunds.create(params, { idempotencyKey });
+};
+
+export default {
+  refundCharge,
+};
diff --git a/modules/billing/tests/billing.pricing.config.wiring.unit.tests.js b/modules/billing/tests/billing.pricing.config.wiring.unit.tests.js
new file mode 100644
index 000000000..64253425a
--- /dev/null
+++ b/modules/billing/tests/billing.pricing.config.wiring.unit.tests.js
@@ -0,0 +1,17 @@
+import { describe, test, expect } from '@jest/globals';
+import defaults from '../../../config/defaults/development.config.js';
+
+describe('config.billing.pricing wiring (development.config.js):', () => {
+  test('exposes PRICING_VERSION, PLAN_QUOTAS, RATIOS, STRIPE_PRICE_CENTS, STRIPE_PACK_CENTS', () => {
+    expect(defaults.billing).toBeDefined();
+    expect(defaults.billing.pricing).toBeDefined();
+    expect(defaults.billing.pricing.PRICING_VERSION).toBeDefined();
+    expect(defaults.billing.pricing.PLAN_QUOTAS).toBeDefined();
+    expect(defaults.billing.pricing.RATIOS).toBeDefined();
+    expect(defaults.billing.pricing.STRIPE_PRICE_CENTS).toBeDefined();
+    expect(defaults.billing.pricing.STRIPE_PACK_CENTS).toBeDefined();
+  });
+  test('PLAN_QUOTAS has at least `free` numeric entry by default', () => {
+    expect(typeof defaults.billing.pricing.PLAN_QUOTAS.free).toBe('number');
+  });
+});
diff --git a/modules/billing/tests/billing.refund.service.refundCharge.unit.tests.js b/modules/billing/tests/billing.refund.service.refundCharge.unit.tests.js
new file mode 100644
index 000000000..ec195f5b8
--- /dev/null
+++ b/modules/billing/tests/billing.refund.service.refundCharge.unit.tests.js
@@ -0,0 +1,61 @@
+import { jest, describe, test, expect, beforeEach } from '@jest/globals';
+
+describe('billing.refund.service — refundCharge:', () => {
+  let RefundService;
+  let mockStripe;
+  let mockRefundsCreate;
+
+  beforeEach(async () => {
+    jest.resetModules();
+    mockRefundsCreate = jest.fn().mockResolvedValue({ id: 're_test_123', status: 'succeeded' });
+    mockStripe = { refunds: { create: mockRefundsCreate } };
+    jest.unstable_mockModule('../lib/stripe.js', () => ({
+      default: jest.fn().mockReturnValue(mockStripe),
+    }));
+    ({ default: RefundService } = await import('../services/billing.refund.service.js'));
+  });
+
+  test('initiates a full refund when amountCents omitted', async () => {
+    const result = await RefundService.refundCharge('ch_test_123');
+    expect(mockRefundsCreate).toHaveBeenCalledTimes(1);
+    const callArgs = mockRefundsCreate.mock.calls[0][0];
+    expect(callArgs.charge).toBe('ch_test_123');
+    expect(callArgs.amount).toBeUndefined();
+    expect(result.id).toBe('re_test_123');
+  });
+
+  test('uses partial amount when amountCents provided', async () => {
+    await RefundService.refundCharge('ch_test_456', 5000);
+    expect(mockRefundsCreate).toHaveBeenCalledWith(expect.objectContaining({
+      charge: 'ch_test_456',
+      amount: 5000,
+    }), expect.any(Object));
+  });
+
+  test('passes deterministic idempotency key derived from charge+amount', async () => {
+    await RefundService.refundCharge('ch_idem_test', 1000);
+    const idemArg = mockRefundsCreate.mock.calls[0][1];
+    expect(idemArg).toBeDefined();
+    expect(idemArg.idempotencyKey).toBeDefined();
+    // Same charge+amount → same key (deterministic)
+    await RefundService.refundCharge('ch_idem_test', 1000);
+    const idemArg2 = mockRefundsCreate.mock.calls[1][1];
+    expect(idemArg2.idempotencyKey).toBe(idemArg.idempotencyKey);
+  });
+
+  test('passes reason when provided', async () => {
+    await RefundService.refundCharge('ch_test', undefined, { reason: 'requested_by_customer' });
+    expect(mockRefundsCreate).toHaveBeenCalledWith(expect.objectContaining({
+      reason: 'requested_by_customer',
+    }), expect.any(Object));
+  });
+
+  test('throws when stripeChargeId is empty', async () => {
+    await expect(RefundService.refundCharge('')).rejects.toThrow();
+  });
+
+  test('throws when amountCents is 0 or negative', async () => {
+    await expect(RefundService.refundCharge('ch_x', 0)).rejects.toThrow();
+    await expect(RefundService.refundCharge('ch_x', -100)).rejects.toThrow();
+  });
+});