Real-Time Shipping Error

We are getting many email notifications as follows:

Customers have experienced issues with receiving rates from the USPS shipping method. Please make sure that is configured properly and that the shipping rate calculation is correct.

The info is completed under Settings > Checkout > Default customer location.

Tested successfully for shipping services under Administration > Shipping methods.

There are no entries regarding these errors under Administration > Logs.

How to troubleshoot it? Thank you.

I'd like to help you, but so far all you can do is to turn OFF logging for "Realtime shipping errors" in the Settings>Logging form.

CS-Cart can't explain it. No one can find the error. It's been broken for years.

I have purchased one customize gift from a webiste and the ordered quantity was saved in it. Next time when I buy the good that auto saved quantity added to my purchased queue