I would think you could easily dispute that. I'm pretty sure you can only be charged what you authorized, even in this case when you've not used their coupons as they wished. Of course, they could then say only one coupon could be used, so you may have to return the drive or face their legal action. I would first dispute the extra charge. I don't think they have a leg to stand on there. At that point, they would probably ask you to return the drive for the entire credit back. If you didn't, I guess they could sue you for it, but the extra charge should be approached differently than the original purchase and whether that was valid. They'll try to say it's one purchase, but if it shows up as two charges...insist (in writing with your paper work), that second charge wasn't authorized.