The steps of using WeChat to scan postal QR code for payment are as follows:

1. First open the WeChat APP, click the+sign in the upper right corner and select "Sweep".

2. After entering the code scanning interface, click "I want to pay" and select "Pay QR code".

3. At this point, open the payment code of the postal merchant to be paid, and click "Sweep" to complete the payment.

Please note that you need to ensure a good signal and avoid using public networks when using postal receipt codes for transactions. In addition, if there is no payment, it is recommended to check whether it is a network problem or whether the payment amount exceeds the limit.