We don't send in extra money to be used as a credit, but we do always send in enough money to have our room (and tickets, if we don't already have them) paid in full before we check in. It just makes things easier for us. We like to have it all paid and ready to go when we arrive. We always do this. Twice we have had to cancel or change our plans, and we have never had any problems at all getting money back. Once, we changed resorts and they just transferred the money over to the new resort. Once, we had to cancel and they just sent us a check for the full amount we had paid. It took a few weeks to process the refund, but we did get it without any problems. Just make sure you keep careful track of what you send in. Keep your cancelled checks as proof, etc. and you shouldn't have any problems.