We just used XRT Golf Carts (you can find them on Facebook), while we were there for the weeks of Christmas and New Year's. We paid $250 US per week. Mohammad was super-nice - met us at the ferry port (we came in via Chetumal ferry), and when I went to pay him (he had asked for cash), he asked, "Is that all of your cash?" - seeing that it obviously was all the cash I had in my wallet. I said, kind of hesitantly, "Yes..." He said, "Here -" and handed me $40 US back. "You'll need money for the bridge (he knew we were staying north of the bridge), and in case you need it for something else. That way you don't have to waste time tonight going to the ATM. You can just bring me it sometime this week."
It was so nice! We hadn't even thought about the toll bridge and needing cash!
We did have an issue with the cart not starting a few days later, after dinner. We called him, and he came over right away and got us another cart, no problem whatsoever.