I'm pretty sure this has already been discussed but we generally tip $1 a bag in the U.S., which works out to $2 for us, and I notice flight crews ordinarily doing about the same FWIW. We do the equivalent in Europe and the UK although we don't seem to find as many shuttles there.

Oddly enough I rarely tip rental-car shuttle drivers, perhaps because they are ordinarily shorter trips and we're carrying our own bags on and off.

Recently a shuttle driver at an Italian hotel voluntarily drove us several minutes out of the way past the regular stop to drop us at the Rome Termini, which otherwise would have required a subway or cab, complete with our roll-aboards. While I realize he was driving the hotel's bus and being paid, I still appreciated his efforts and gave him 5 Euros, which seemed to genuinely surprise and please him.