R 7-50 Proof of purchase

I made some keys at Wardkiss Hardware in Umbilo recently, and also purchased an MH12 key over the counter. I did not check the number at the time, but when I needed to use it, it did not open the door lock - that is when I realized that they actually supp**** an M12H key. This morning I was in the area and decided to exchange the R7.50 key with them, but the store did not have any in stock. The manager offered to order one, provided I had the original till slip. I said that I did not, and suggested a refund, and again was refused for the same reason. I can understand the logic behind requesting a till slip as proof of purchase, but he should understand that cars do not run on fumes, and it would cost more in petrol to fetch the proof of purchase than what the actual item was worth. Quite sarcastically he agreed to destroy the key with a bolt cutter for me, when I made the suggestion