Whatever you do keep in mind you can only book one ticket on a pairing. That is one of their rules.

Try to find that one in the instructions or in the contract...

Maybe that World (Douche) Guy can show you where that is in the FDX contract

Is this a domestic rule? I do it all the time on international. I think I've done it in years past on domestic.

I have done that internationally as well. I think it only works with a d/h to layover to another d/h. I called yesterday and was told no, you can't book a domestic layover. It has to be two separate tickets, which would exceed my bank by a lot... :(