Hi,
In February 2025, USPS announced it would retire its WebTools API in January 2026:
postalpro.usps.com/node/13834
So, while the API of the USPS plugin should still work for 1 or 2 months, maybe the access to the WebTools API has been cut faster than expected for your client's account ?
We've made a new plugin 6 months ago for the new API:
www.hikashop.com/marketplace/product/290-usps.html
We've made important updates to it during summer thanks to feedback from numerous merchants so that it can support everything the old plugin supported.
Migrating to the new plugin might be the best at the moment.
The new plugin also comes with better debugging than the old one, so it will be easier to know what's going on if there's a problem.