That is the first issue that comes to mind. I can't even think of a way you would fix it. You could make an agreement with other companies to use the same coin, but then it isn't guaranteed that the currency will be used in your store, which is the whole point of having in-store currency. You probably aren't going to be able to reach an agreement for that reason, which means your only other option is to just use a normal coin with low transaction fees, but that defeats the point even more because then that currency can be used anywhere.
I hate in-store shit anyway to be honest. Especially when they offer it to you after you trade in a game. It's just an excuse to pay you less.
Sounds like a replacement for vouchers or tokens. A shitty in store currency that ceases to be valid after a year if you read the small print.
You could make an agreement with other companies to use the same coin
I literally worked for a start-up around 5 years ago that tried to do generic reward card systems that had a similar concept. By the time I had the website up and running, 7 other companies were seeking VC funds for an identical concept in the bay area alone. None of them existed after a year, so I'm pretty skeptical of a generic crypto gaining traction.
Not a terrible idea to do crtypto for store rewards. Might get overwhelming when you own literally thousands of types of crypto currency