Domestic (US) customers get offered both Express and Priority, but suddenly, no matter which they choose it switches it to Express. I tried uninstalling and reinstalling and it didn't help. I called 800-ask-usps and they said it is a problem some 3rd party carts are having and it's caused by the cart not requesting rates with all caps.
I'm using:
Zen Cart 1.3.7.1
Database Patch Level: 1.3.7.1
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