interactions between the merchant system and the WeChat Pay system are as follows:

1. The login API is called in the Mini Program to get the user's openid. See [Mini Program Login API]

2. The merchant server calls the order placement API for payment. See [Order Placement API].

3. The merchant server calls the re-signing API. See [Mini Program Calls Payment].

4. The merchant server receives payment notification. See public API [Payment Result Notification API].

5. The merchant server queries the payment result. See public API [Query Order API].