If you go over your bandwidth, you're charged by the GB, based on your plan. L1 plans pay $1 per GB of overage on bandwidth. You can always upgrade your plan before that happens, though!
Regarding the money-back guarantee, the DreamHost Wiki says that it's only for credit cards. Now as you know, when you use your debit card, it can either be a debit transaction (with a PIN) or a credit transaction (without a PIN). I would guess that if you did the latter (which is the only method I think DreamHost supports), you'd be covered. However, it never hurts to check by sending a message to customer support!
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