As long as you return the phone and everything that was in the box, during the return period you are good to go.
If you lost the box, or any of the items and paperwork in the box, you are going to have to pay (what I don't know), but to get out of paying any "fees" you need to return the phone in the same condition you bought it in.
You of course pay for the two weeks of calls/texts if over any plan mins/txt, and maybe a restocking fee.