It's not a problem. The argument is that if they're part of the US government, they're required to release the information requested under the Freedom Of Information Act. USPS is trying to make the case that they're a private business not connected to the US government (and hence, aren't required to comply with the FOIA), which clearly is not true.

It doesn't seem that clear to me. Especially considering the tax part. I'm thinking there's probably stuff the USPS knows that really shouldn't be public information.