Just a thought....
I have a rebate prepaid VISA card I plan to use this trip (leaving tomorrow!). On the information sheet that came with the card, it states that the merchant may try to authorize 20% of your bill to cover gratuity. Therefore, I will not use it to purchase food or services; only merchandise.
I have had a problem with these types of cards in the past. For example: several years ago I went to a nail salon and used a prepaid VISA card to pay my bill. 20% of the value came off of the card automatically as gratuity without my knowledge. My advice to you is read the fine print BEFORE you purchase the cards.