If you've owned it for more than 15 days, you own it for good. If been less than 15 days and you decide you want a more expensive Mac, I think they will wave the fee. I think if you just want to return it, less than 15 days, they will definitely charge you. Look at it from their perspective, they now have to sell as a refurbished MB.
Just curious to why you would want to return it??? That could factor in to the decision. Good luck.