It is there standard practice, one is for the real rental price in case you bring it back late, the other is for damage done to the car and you don't have time to get it adjusted before leaving the island, they have your signature on file. When you bring the car back, if there is no damage, they will tear up the unused credit card slips, and if you want, they will give it to you. They will also give you a copy of the rental receipt when you bring back the car also. I personally have not had a problem with them in the 6 times that I have used them. Besides, if you had a problem with excess billing, you can always dispute the charges with your credit card company, but I never had that problem along with many others on this board.