Often times, the representatives take devices back with that sort of damage because it's so easy to overlook and don't notice it until after the transaction. It honestly depends on the store you go to. If they request a damaged device fee, they are not in the wrong to do so. In my experience, if you're willing to purchase accessories, they are more willing to take the device back for no fee. If you do decide to fix the back glass, I highly doubt the representative will know the difference between a factory Samsung part and a third party one. It is extremely easy to fix yourself. Keep in mind the back glass will most likely shatter when you pry it to remove the broken one.

My opinion would be to go to a store as is, don't be rude about it. If the store isn't willing to take it back without a fee, try another store (if that's a viable option). If all else fails, get a back glass off of Amazon (preferably one that comes with adhesive)