I wouldn't pay storage fees but then again wouldn't expect someone to hold it for me that long, didn't know people did that. If they pay in full give them the option of having it delivered for a fee if you can just to get it out of the store.
Might make them get their tails in gear and get someone to take it out of your business. Some people don't care how much they put someone out. It could be that they have to wait for their son to come home for the weekend to help out or whatever. You might lose a sale if you impose those fees though or make them wait, they could change their mind about the purchase.