Hi,
I would recommend phoning someone at USPS and see if they can redirect you to the correct person.
Here is a list of a lot of contacts at USPS, with their phone numbers for many:
postalpro.usps.com/support
For example, the guys at the "Parcel Data Exchange (PDX) API" might be colleagues of the ones for the webtools API.
Now, the error message being "User xxxxxxLANGExx is not authorized to use API RateV4", it sounds to me that your USPS account needs to have something activated to be able to use the API.
And looking again online about it, I can see another page talking about it, also mentioning to contact USPS to have it activated for your account:
wordpress.org/support/topic/api-authoriz...d-to-use-api-ratev4/
So I'm afraid we can't do much on our end besides redirecting you to USPS.