The statute on mail fraud (18 U.S.C. § 1341: https://www.law.cornell.edu/uscode/text/18/1341 https://www.law.cornell.edu/uscode/text/18/1341) clearly states: >

The statute on mail fraud (18 U.S.C. § 1341: https://www.law.cornell.edu/uscode/text/18/1341 https://www.law.cornell.edu/uscode/text/18/1341) clearly states: > places in any post office or authorized depository for mail matter, any matter or thing whatever to be sent or delivered by the Postal Service, or deposits or causes to be deposited any matter or thing whatever to be sent or delivered by any private or commercial interstate carrier (Emphasis mine). It is clear from this that committing fraud via FedEx and UPS would be treated the same as USPS. The one difference, AIUI, is that the USPS is protected by the US Postal Inspection Service (who are, as you say, quite zealous), whereas I’m not sure the same extends to UPS/FedEx (where you’d probably be looking at an FBI investigation instead). However if you (or Amazon) mailed e.g. the return labels over USPS the USPIS could still get involved.