Extremely unlikely. I get more thoroughly vetted traveling on Fedex than I do any passenger carrier. Show me in the history of Fedex/UPS asides from Calloway when a stowaway hijacking has happened?
Risk calculation.... where would you put that chance? I'd have to put it zero. What terrorist going to try to stowaway on a Fedex plane in hopes of trying to take you over? Never has happened. Far easier to just outright buy a used 727/737 on the market (ASO.com) and fly it as a weapon.
Inside threats, yes. Stowaway or hijacker argument on Fedex/UPS gets to be ridiculous. If we are to be reasonable and honest, your biggest threat in terms of humans is going to be the insider factor. And as history has shown, that's *precisely* what it was.
FFDOs on Fedex/UPS is a laughable concept.