There must be a way to easily s/e codes from them.
Going to have a look at their site and see if theres a web support chat.
Update: went on the frito lay site and talked to their web support saying I bought a bag of doritos and the code didnt work etc, they gave me a phone number to call regarding the issue. I dont live in the US so cant really call them because it would cost a bomb lol but im sure if you contact their customer support and say you bought a bag and the code has already been redeemed you can get free codes aslong as you are convincing, dont pay for this shit unless someones selling it dirt cheap... good luck.