The Cart HTML5 interface is a pre-created checkout interface on top of our API. It is white-label and designed as mobile-first.
To use it just point a web browser, a UIWebView (iOS), or a WebView (Android) to https://checkout.twotap.com with a pre-created checkout_request.
The Cart requires you to implement a server side process for confirming orders which talks to Two Tap's API. You can choose between the SMS flow (where Two Tap handles shopper communication in case of issues like invalid coupons) or the HTTP flow (where you have to take care of it).
The Cart interface is stateless, which means you can't add products to an existing cart. For multi-products / multi-retailers we recommend you store the list of product URLs locally (localstorage, NSUserDefaults), and open the interface once the user taps your checkout button. You don't have to do any extra UI work since you would have a 'checkout' button (that contains a count of products in the cart) anyways. When shoppers press that button open Two Tap with that array of product URLs.