I am sorry for causing confusion. I had meant to say United Parcel Service (The commercial carrier). Your perspective concerning not-for-profit is valid and duely noted.
I would like to know more about your experiences interfacing with the United States Postal Service (Federal). They are also on my list of interfaces to look into.
Actually - my "interactions" are few and far between. Mostly I just parse addresses so I can cross reference people from multiple databases. Some other people have said they'd start working with the USPS's SOAP interface but I haven't seen that materialize yet.