Enabling USPS Causes Checkout Error

I have a Power Store. When I enable the USPS shipping, the user is getting an error:

Transaction Not Completed
We're sorry. Your transaction could not be completed. Either your credit card information or billing address has problems and cannot be processed
Please return to checkout and try again.

When I do not enable this, the checkout works fine. Apparently there is an authentication error with USPS's API. Can you help me please? Attached are the credentials you will need.

I enabled shipping and it gave me a rate. What did you have in your cart at the time? Please try again. I added debug information to the failure page we can use to diagnose the cause when it happens again.

I see the debug you inserted. When I disable the USPS integration, my credit card goes through fine. But when I enable the API, I get the error below when I checkout buying a Nike Dri FIT Golf Shirt for$53.00 Size: Men - Small Color: : White is as follows:

We're sorry

Your transaction could not be completed. Either your credit card information or billing address has problems and cannot be processed

Please return to checkout and try again.

The error is:

The requested Mail Service is only available when Ground Transportation is required, when the Zone is 5-9, or the mail piece is Oversized

Which explains why it worked for me. I'll look into it further now using your address.

You have the default mail type set to "First Class" and first class mail is only available for packages under 13oz.... The code detects that and sets the mail type to "PARCEL" when it is oversized for first class and first class is selected. Well it turns out that PARCEL also has restrictions as identified in the error message.

I've updated the code to use "PRIORITY" when first class is selected and not available. That is the standard USPS mail type and it should be available everywhere.

Thank you Ray! It works great. I have a PS 4.5 cart that I will be using USPS with. Is there some edits I should apply to that site also to address this USPS issue?

You can update the WA_eCart/WA_eCart_USPS_PHP.php file. However another solution would be to simply choose "PRIORITY" instead of "FIRST CLASS" as your mail type, since I don't think you have items under 13oz in your store anyway.