Thermal paper isn't the only problem (Score 1)

I once bought a VCR, which had the typical 1-2 year warranty from K-mart (similar to a walmart in the US, I'd imagine). After, I shit you not, one night of being left in the rain (not the VCR! The receipt) outside, still attached to the box (I'd forgotten to remove it) the receipt was totally blank. Even a forenzic scientist would not be able to retrieve the information from it, much less a store clerk. With regards to thermal printed receipts, well - as many have already said, the only way to keep those is to photocopy them. Of course, keep the original anyway; it should still be at least partially readable, and should provide further proof that the photocopy is genuine. Sunlight is another great thermal paper killer as well. It's much the same as with thermal faxes, they're notoriously dodgy, especially where the information on the document is important.