> The release of the existence of such a relationship or of the material terms of any such agreement, should it exist, with a particular business entity could harm the Postal Service's commercial interests in obtaining competitive rates with a variety of business partners in the future.
Understandable reason. Redacting only the "necessary" bits might well render the document unreadable.
Yes, you can submit a FOIA request to the USPS requesting the contract.
https://www.foia.gov/