Here is a note about some really nice folks - the staff at the Gasolinera Maya Pemex station on the highway to Progreso (the one that has a crappy 7-11 on the premises).
This gas station has always struck me and my better half as being leagues above the norm in customer service. When you arrive at the pump, the greet you with a smile and "welcome to the Gasolinera Maya" and they actually look like they are happy to see you. They are also one of the first gas stations with a remote credit card terminal so you can pay with debit or credit cards from the comfort of your vehicle.
Yesterday, I filled up one car and paid with my debit card. 2 hours later, I stopped by again with a second vehicle and had that one filled up as well. When I tried to pay with the debit card, there was a communication problem with the terminal and, after two attempts, I tried with the credit card. None went through. Not having any cash on me and with no ATM around, I half-jokingly said, "well, you know who I am, I can come back later and pay you". And guess what? That's exactly what happened! These people just asked me to sign a small slip of paper, leave my name and number and to come back later when the terminal was working again!
I have got to say I was blown away by their confianza and goodwill! I would rate their customer service, on a scale of 1 to 10, at a solid 10. Don't subject yourself to indifferent or bad service (Servicio Campestre, across from Sam's is a consistent and depressing -1 on the service scale, for example) by buying your gasoline anywhere else!