Other than a warm body, during our peak, why would FedEx want to hire you? Granted those jobs are not the most complicated, but by your taking one of them, with the motive of leaving in a month, you will certainly not ingratiate yourself with your manager, who will, after only a month, have to go out and re-interview for the job you're leaving. As well, if you take the job, without telling FedEx that you plan on leaving on military leave in January, they will certainly remember your little stunt.

A better course of action is to be up-front with them and if they really want you for the short duration that you can give them, they'll hire you. If not, they will at least recognize your honesty and remember that about you.

Regardless, best of luck and enjoy UPT.