A simple code verification gateway script or plugin

SO here’s my dilemna…

I need to have this page: http://shop.chooseorange.org be able to have ONE CODE at a specific length – be input and verified before you (user) are allowed to gain access to the next page, where you would put in all of your info.

Users will buy the code offline and then use the code to order their product. The codes are already predetermined so the plugin or script would have to be able to read against a csv or database AND mark it as used.

I, for the life of me, can’t figure this out – and it needs to work like yesterday.