I would speak with the manager of the store, ask the person if they would give you a deal or trade on some fry. They may offer you a small amount of cash or possibly even store credit. Store credit more than likely but you never know.
Petco, Petsmart, and most other big box stores won't accept guppy fry because they have contracts with they're sellers saying they can only buy from them. If it isn't a chain store though, you should be good.