> ## Documentation Index > Fetch the complete documentation index at: https://docs.omnicommerce.ai/llms.txt > Use this file to discover all available pages before exploring further. # Start payment for an approved checkout offer > Requires checkout:write. Starts seller Stripe, Qashier or HitPay hosted checkout. Buyer approval must match the current cart version, total and currency. Qashier and HitPay require a full delivery address before payment. Qashier and HitPay support PayNow in Singapore when enabled for the merchant. Payment is confirmed using signed webhooks and authenticated server-side receipts. Closing the buyer page does not cancel or complete payment. ## OpenAPI ````yaml /openapi.json post /api/v1/checkout-sessions/{sessionId}/pay openapi: 3.1.0 info: title: OmniCommerce API version: 1.0.0 license: name: Proprietary url: https://omnicommerce.sg/terms description: >- OmniCommerce API surface for agents and developer integrations. Includes public endpoints (merchant discovery, product search, compare, catalog, ACO retrieval) and authenticated endpoints (product management via API key Bearer tokens). servers: - url: https://omnicommerce.ai description: Production - url: http://localhost:3000 description: Local development security: [] tags: - name: agentic-checkout description: Merchant-fulfilled agentic checkout x-group: Agentic Checkout - name: agents description: agents x-group: Agents - name: analytics description: Sales performance across marketplaces x-group: Analytics - name: aops description: Agent operating procedures x-group: AOPs - name: branding description: branding x-group: Branding - name: campaigns description: Campaign briefs, offer windows, and marketplace runs x-group: Campaigns - name: catalog description: catalog x-group: Catalog - name: commerce description: commerce x-group: Commerce - name: developer-platform description: Authenticated developer platform APIs x-group: Developer Platform - name: documents description: Knowledge base document uploads x-group: Knowledge Base - name: follow-ups description: follow-ups x-group: Follow Ups - name: inventory description: Inventory quantities, planning signals, and marketplace listing absence x-group: Inventory - name: looks description: AI-styled product looks x-group: Looks - name: oauth description: OAuth token endpoints x-group: OAuth - name: orders description: Workspace and agentic orders x-group: Orders - name: organizations description: Organization management x-group: Organizations - name: price-books description: Marketplace list-price markup rules x-group: Price Books - name: products description: Product CRUD, bulk import, enrichment, publish, and delist x-group: Products - name: promotions description: Central promotions and marketplace sync x-group: Promotions - name: public-agent description: Public agent discovery and catalog APIs x-group: Public Agent - name: returns description: returns x-group: Returns - name: reviews description: Marketplace product reviews and seller replies x-group: Reviews - name: scripts description: scripts x-group: Scripts - name: settlements description: settlements x-group: Settlements - name: ucp description: Universal Commerce Protocol x-group: UCP paths: /api/v1/checkout-sessions/{sessionId}/pay: post: tags: - agentic-checkout summary: Start payment for an approved checkout offer description: >- Requires checkout:write. Starts seller Stripe, Qashier or HitPay hosted checkout. Buyer approval must match the current cart version, total and currency. Qashier and HitPay require a full delivery address before payment. Qashier and HitPay support PayNow in Singapore when enabled for the merchant. Payment is confirmed using signed webhooks and authenticated server-side receipts. Closing the buyer page does not cancel or complete payment. operationId: post_v1_checkout_sessions_by_sessionid_pay parameters: - name: sessionId in: path required: true schema: type: string requestBody: required: true content: application/json: schema: type: object properties: approval: type: object properties: cartVersion: type: integer exclusiveMinimum: 0 maximum: 9007199254740991 totalAmount: type: number exclusiveMinimum: 0 currency: type: string pattern: ^[A-Z]{3}$ required: - cartVersion - totalAmount - currency additionalProperties: false required: - approval additionalProperties: false responses: '200': description: Hosted checkout URL. content: application/json: schema: type: object properties: success: type: boolean const: true sessionId: type: string checkoutUrl: type: string format: uri paymentExpiresAt: type: string format: date-time pattern: >- ^(?:(?:\d\d[2468][048]|\d\d[13579][26]|\d\d0[48]|[02468][048]00|[13579][26]00)-02-29|\d{4}-(?:(?:0[13578]|1[02])-(?:0[1-9]|[12]\d|3[01])|(?:0[469]|11)-(?:0[1-9]|[12]\d|30)|(?:02)-(?:0[1-9]|1\d|2[0-8])))T(?:(?:[01]\d|2[0-3]):[0-5]\d(?::[0-5]\d(?:\.\d+)?)?(?:Z))$ required: - success - sessionId - checkoutUrl - paymentExpiresAt additionalProperties: {} '400': description: >- Invalid approval, unsupported payment currency or missing delivery details. '403': description: Checkout disabled or organization access denied. '409': description: Offer changed, payment not configured or offer no longer payable. '503': description: Gateway unavailable; retry the same offer. security: - bearerAuth: [] components: securitySchemes: bearerAuth: type: http scheme: bearer bearerFormat: API Key description: >- Bearer API key for server-to-server access. Session auth is also supported in first-party UI flows. ```` This documentation is built and hosted on [Mintlify](https://mintlify.com), a developer documentation platform.