I guess all Nans must be the same. I took my Mum and Nan out for lunch. The meal was super and so was the service. I left a couple of pounds on the table. When we got to the car my Nan put two pound pieces in my hand and said "Here, you left these on the table". I was absolutely mortified and rushed back to give the waitress the tip...hand to hand!
But back on topic, I know many passengers in the US will give deserving crew (probably most of you
) a loaded Starbucks card. Would that kind of thing be acceptable? After all, it's not cash is it?