Development Guide
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].
