Just found this excellent post on the concept of Tipping. The (opt-in) ‘patronage’ aspect of tipping is important. The challenge is always going to be how to aggregate small amounts of money to avoid drowning in credit card fees. PayPal is one (not very popular) option.
My thought would be that people would purchase ‘tipping credits’ like $10 or $20 amounts (like one does on Skype), hence only causing one credit card transaction (fee).
The issue is whether people would be willing to spend that kind of money on ‘future’ tips. I think its worth a go.