The folks at Stamps.com reached out to me. Well... it's fixed. The First Class and Priority now is correctly retrieved by Stamps.com when I use the Stamps.com Batch selection.

The only thing is that is works very well with the International selections (since there are so many in the USPS module for Zen-Cart) however, the Domestic selections are just very generic like:
EXPRESS
PRIORITY
FIRST CLASS
PARCEL
MEDIA
BPM
LIBRARY

Whereas International has this:
Global Express
Global Express Non-Doc Rect
Global Express Non-Doc Non-Rect
USPS GXG Envelopes
Express Mail Int
Express Mail Int Flat Rate Env
Express Mail Int Legal
Priority Mail International
Priority Mail Int Flat Rate Env
Priority Mail Int Flat Rate Small Box
Priority Mail Int Flat Rate Med Box
Priority Mail Int Flat Rate Lrg Box
Priority Mail Int DVD
Priority Mail Int Lrg Video
Priority Mail Int Legal Flat Rate Env
Priority Mail Int Padded Flat Rate Env
Priority Mail Int Gift Card Flat Rate Env
Priority Mail Int Small Flat Rate Env
First Class Mail Int Lrg Env
First Class Mail Int Package
First Class Mail Int Letter

I would love to see:
Priority Mail Padded Flat Rate Env
Priority Mail Flat Rate Small Box
Priority Mail Flat Rate Med Box
Priority Mail Flat Rate Lrg Box
to name a few...

Any ideas how I would add these to the current USPS module?

In summary: Stamps.com integration works well now!