For a trip back to the parking lot, I add another 4 or 5 dollars to the tip, and we have 3 suitcases and two carryons, so it's about 12 dollars. If it's raining or very humid, I add more to the tip so it gets up to $15. It definetly takes a lot more effort to get to the parking lot than from the dropoff area to the ship.
No way to prove this, but it seems to go a lot smoother thru the Customs people with your bags stacked on one of the porter's little cart. We never buy that much and no tobaco or alcohol, but I would just like to avoid a delay, if possible. It just seems like the porter is there and nodding to them to say 'hey, they're probably Ok, I didn't see anything unusual'.