I like Mega a lot, just beware of timeshare sharks in there, pretending to help you find the products you are seeking. We really enjoyed the open air market in the far part of their parking lot.
There is a Costco and we found out the hard way that they do not accept Costco Cash Cards.
For credit cards, call your issuers to see which, if any, skip transaction fee. I would think that preferable to opening new account for just this trip.
Karen's suggestion about checking on kitchen facilities first is very very good advice. You also might check on whether your resort has a small grocery/deli. You'd pay more, but the convenience is nice.