a bit elaborate, but it works well
me and my mate did this the other day:
1. get small bin (might be tricky) and put it next to a shop door. We put a sign on it saying "put your old receipts in here, don't litter!" but you probably don't have to
2. wait a while, then come back and get the receipts without arousing too much suspicion.
3. go into the shop, perhaps with a bag, and get "refunds" for the more expensive stuff that you "bought", using the receipts.
The store can't really not refund you, unless they have proof that you didn't buy the item. We did this in a couple of stores with a small bin we found, and made nearly £100
