Sorry for being quiet on this for so long, but unfortunately, there's much more to it.
Firstly, I had to invest a lot of time in new requirements to release updates of apps on Google Play. I'm close to finishing that.
Secondly USPS seems to be going into a very unfortunate direction that will keep me from offering them as an easy-to-use option within the app starting next month, to quote:
Paid Consumer Service Provider Access
USPS will provide paid access to the Tracking API for any Mailer ID through the USPS API platform. This is for companies that do not play a direct role in the creation of a package label but may provide information to end consumers. Paid access will be subject to a monthly fee and other requirements set forth in a signed Intellectual Property (IP) license agreement. Access to track packages for MIDs which are not owned by or authorized to the third-party provider is limited to companies with a signed IP agreement under the specific conditions noted in the agreement.
Full details: https://postalpro.usps.com/Tracking_APIAccess_Control_Changes_Release_Overview
So even if the monthly fee would be feasible to be paid by me (I doubt it), it seems like it wouldn't even work for a lot of packages. On top of that, the USPS developer portal won't grant me access to creating API credentials with a newly created business account,...
I will still try to find a solution, but it looks a lot like it will have to be moved to a non-native integration as soon as the old API stops working again 
