Don’t think that matters as far as criminality goes. FedEX could have cooperated with the law enforcement just as well. Mail and wire fraud doesn’t mean only USPS and only metal wire. They are broad enough to encompass inter-state fraud as much as they can.
If they had used FedEX or UPS this wouldn't apply or would it? What they did was fraud but that may not be as severe as mail fraud.
https://www.law.cornell.edu/uscode/text/18/1341