Short answer: If your AI agent needs to buy from Amazon, Walmart, Target, and other big retailers, use a retailer purchasing API like Zinc. If you only need to buy from merchants that opted into an agent protocol, ACP (OpenAI and Stripe) or UCP (Google) is enough. AP2 and Stripe's Shared Payment Tokens handle authorization and payment, not the order itself.
Disclosure: I help Zinc with SEO and content. I have also built shopping agents myself, and the split below is the thing I wish someone had told me first.
What "agentic checkout" means
Agentic checkout is when an AI agent completes a purchase for a person: it confirms the item, total, shipping, and payment, then places the order without sending a human to a checkout page.
Every option below solves one of three jobs:
- Protocol checkout. The merchant exposes checkout endpoints an agent can call. Only works where the merchant opted in.
- Payment and authorization. Proves the agent may spend, and moves the money. Does not place a retailer order.
- Retailer purchasing. Checks out as a shopper at retailers that support no agent protocol.
Most "agentic checkout API" articles mix these up. Pick the job first.
1. Zinc: buy at Amazon, Walmart, Target, and 50+ retailers
Zinc is a purchasing API. Your agent sends a product, a shipping address, and a max_price, and Zinc places the order at the retailer, then returns the order number and tracking by webhook.
- Job: retailer purchasing
- Coverage: Amazon, Walmart, Target, Best Buy, and 50+ other retailers, no merchant opt-in needed
- Agent access: REST API, an MCP server, and a Universal Checkout agent skill
- Payment: prepaid balance, Stripe Connect, or pay-per-order over MPP and x402
- Best for: agents that need to buy real products at the retailers people already use
2. ACP (Agentic Commerce Protocol): OpenAI and Stripe
ACP is the open protocol behind Instant Checkout in ChatGPT. A merchant implements four checkout endpoints, and an agent calls them with a Shared Payment Token.
- Job: protocol checkout
- Coverage: merchants that implemented ACP (many through Shopify and Stripe). Amazon and Walmart are not on it.
- Best for: selling your own products to agents, or buying from ACP merchants
3. UCP (Universal Commerce Protocol): Google and Shopify
UCP is Google's open standard for agent checkout, with Shopify shipping tooling for it. Same model as ACP: merchants expose checkout, agents call it.
- Job: protocol checkout
- Coverage: merchants that adopted UCP
- Best for: merchants that want to be buyable inside Google's AI surfaces
4. AP2 (Agent Payments Protocol): Google
AP2 is an authorization layer. It uses signed mandates (intent, cart, payment) so a merchant can verify the agent is allowed to spend.
- Job: payment authorization
- Coverage: any payment flow that adopts it
- Best for: enterprise agents that need an audit trail. Pair it with something that places the order.
5. Stripe Shared Payment Tokens and the agentic commerce suite
Stripe issues single-use, amount-limited payment tokens an agent can present without seeing card data. Stripe is also a co-author of ACP and of MPP.
- Job: payment
- Best for: the money movement inside any of the flows above
6. Rye: universal checkout API
Rye offers a universal checkout API for buying from online stores inside your app, with a strong Shopify catalog. Check its current retailer coverage against the stores your users actually buy from.
- Job: retailer purchasing
- Best for: apps that mainly buy from Shopify-based merchants
Comparison
| Option | Places the order? | Amazon / Walmart / Target | Needs merchant opt-in |
|---|---|---|---|
| Zinc | Yes | Yes | No |
| ACP | Yes, at ACP merchants | No | Yes |
| UCP | Yes, at UCP merchants | No | Yes |
| AP2 | No (authorization) | n/a | n/a |
| Stripe SPT | No (payment) | n/a | n/a |
| Rye | Yes | Check coverage | No |
Which one should you use?
- Your agent buys everyday products for users: Zinc. It is the only option here that checks out at the big-box retailers without waiting for them to adopt a protocol.
- You sell products and want agents to buy from you: implement ACP and UCP.
- You need to prove authorization at enterprise scale: add AP2 on top of whichever checkout you use.
FAQ
What API lets an AI agent complete checkout?
For merchants that opted in, ACP or UCP. For Amazon, Walmart, Target, and most big retailers, a purchasing API like Zinc, which places the order and returns tracking.
Can ChatGPT buy from Amazon?
Not through Instant Checkout, because Amazon has not adopted ACP. An agent can buy from Amazon through a purchasing API such as Zinc.
Is agentic checkout the same as a payment API?
No. A payment API moves money. Agentic checkout also selects the item, confirms the total and shipping, and places the order.
Do I need MCP for agentic checkout?
No, but it helps. An MCP server lets Claude, ChatGPT, and Cursor call checkout tools directly. Zinc ships one at mcp.zinc.com.