Keep your receipt for Everything!.
I usually buy a backup ink cartridge so when mine is empty I'm good to go. This time I decided to try a refill from Ofice Max. Never thought about keeping that receipt! When I went to install it a couple months later my HP printer would not accept it. I thought the manager would give me credit towards a real cartridge, but he was very rude and basically said it was my problem without a receipt even though they could test the refill and see it was still full! So much for custonmer satisfaction. There wasn't even a discalimer stating it might not work!