You should also bear in mind the CDW considerations. If you plan to decline the coverage from the rental company and rather rely on the protection from your credit card, most cards require that the ENTIRE RENTAL be charged to the card to provide you a replacement of CDW. Whether using the same card to buy these certificate and at the counter would be considered as charging the entire rental to the credit card is a question to ask either at discoveramerica.com or by calling your credit card's phone number. Could be that these certificates are simply considered the same way as other discounts like coupon codes, etc. Then it should be fine, I presume.