I took advantage of a Wal Mart deal for a 4s when they were out.
It's the same as just doing it with your carrier (EXCEPT MUCH MUCH CHEAPER!).
They handle most carriers and by the time I got home the phone was working (15 mins away).
That said the process took two hours. They had to cater to customers outside the wireless deal. Sam's may be different since they have dedicated people for just phones and not all electronics, CD's, DVD's, Video Games, Consoles, TV's
I ended up ordering two then. I can't remember the pricing difference but it was at least $100 per phone.