I can't send it back and I can't get a mid tower. Is there really no PSU I can put?
NOPE, there is really nothing you can do with it, the whole system is highly proprietary.
You can't even take the motherboard out and put it in another case because it's a BTX motherboard, won't work with any case today.
So the best thing you can do is get another PC, and NOT an SFF no matter what you do.