A week long fest like this probably isn't the best idea. If you've been eating clean for a long time, and then eat like crap you will immediately have issues with cramping, bloating, gas, etc. All of the things that might make your Cancun trip not much fun, not to mention the water down there. I would splurge, but ease into it to see how it treats you. I would eat the crap in the morning to, and not late in the day. Give your body a chance to digest before you sleep.
Also- wouldn't it be better the get your empty calories on alcohol than snack food?