Lots of companies do this. I'm with Mobileshop and they have 4 vouchers over the year and you need to send a specific bill within a tight time window. I am always really careful and send it by recorded delivery so I know exactly when they received it just in case. I've not had any problems yet but I won't be going with them again.

They think that a lot of their customers will be chavy little rat boys who won't remember but it sounds like you've done everthing they asked. Feel sorry for you.