At the bottom of the order form it says by placing an order they are agreeing to the terms and conditions.
If they didn't read them simply refer them back to it, if they didn't read before submitting it's their own fault, a check box doesn't make any difference as they'll do what we all do and just click it anyway.
Come on hands up who has ever read a microsoft terms and conditions agreement? How many have even read the x-cart install one?
Human nature is to ignore the small print until you do something that required you to read it.