We tipped the driver that picked us up $20. He did a grocery stop, helped get the luggage off the belt (We had 6 pcs PLUS golf bags) and then got it all loaded into the trunk so we didn't have to sit with any of it.
The driver that brought us back wasn't as good. We only tipped him ten. He had the same size car, but couldn't get all the luggage into to trunk so we ended up putting a piece in the front seat, and we had one less tote since the bag I used to carry food down was now empty and packed away in another suitcase. Not a big deal really, but we just thought the driver that picked us up did more for us, so we tipped him better.