I just spoke to a rav about the whole taus akum thing. He told me that even if you have a receipt and you purposely hold it back from the store in order to get more, that is still muttar. The only way it would be a problem is if you lets say bough two pairs of the same shoes, one on sale and one full price. If you go to return the sale one with the full price receipt, there since you are proactively misleading them that is assur. Otherwise, it is their problem.