You should consider whether such a reward is actually rewarding and to whom. I think refunds would only apply to small piece of the playerbase. This is unlike ip boosts, which only disappoint the players that don't know what to do with all their ip.
On top of that, refunds aren't really something you need in the first place. I don't care whether I have 0 or 18 refunds, I don't need them because I do some research before making a purchase.
It'd probably be fine to just give people a refund a year or so. Don't give it to reward players for not being negative though, most 'rewarded' players will just feel sad they didn't get something they could actually use.