Would UPS or FedEx mail ever be cared on commercial flights Ex: Southwest, Frontier?
Isn’t that why there’s a U.S flag on the tail?
Yes. Mail is carried on commercial flights. It doesn’t fly for free because of it being a US flagged carrier, either.
Don’t know about FedEx but I do know UPS uses other airlines for freight. Quite a bit of the freight out of the far east is carried by EVA, Qantas, Cathay Pacific, etc.
By the way, UPS and FedEx are both commercial airlines. I believe you meant to say passenger and other cargo airlines.