> For the complete documentation index, see [llms.txt](https://docs.tiun.io/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.tiun.io/reference/checkout/how-it-works.md).
# How it works
Call `tiun.checkout({ productId })` and tiun opens a **full-screen overlay** that runs the entire purchase and sign-in experience. You don't build forms, validate inputs, or host payment fields yourself.
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## What the overlay contains
For a subscription product, the overlay walks the user through:
* **Plan details** — product name, pricing, billing interval, and any trial info.
* **Email entry** — the user is identified by email as part of checkout.
* **Payment method selection** — credit/debit cards, PayPal, Apple Pay, Google Pay, PrePaid (tiun credits), and Twint, where enabled and regionally available.
* **Terms acceptance** and a clear pay action.
A **Login** affordance also appears at the bottom of the overlay so a visitor who already has a tiun account can sign in instead of entering email as a new customer.
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## Identity verification
* If the email is **recognized** (returning user), tiun sends an OTP code to the **phone number** already linked to the account.
* If the email is **not recognized**, an additional overlay asks for the user's phone number, then sends an OTP to validate and link it before payment continues.
Checkout authenticates as part of the same flow — there's no separate sign-up step. See [Authentication / How it works](/reference/authentication/how-it-works.md) for the broader identity model.
***
## Managed by tiun
The overlay is owned and updated by tiun: layout, validation, compliance copy, and payment UI stay consistent without changes in your codebase. The flow also surfaces in-overlay help — including an explainer video link and "More info" affordances where appropriate — so common questions are handled inside checkout.
For what happens after the user completes checkout (the journey, session restore, logout), see [Subscriptions](/reference/checkout/subscriptions.md). For the resulting entitlements on the user, see [Product access](/reference/checkout/product-access.md).
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