None
You'd have to be a real asshole (or receive the worst service imaginable) to give someone a disrespectful tip after they've braved the cold to bring you whatever. Some people will (because they're assholes), but it's good to know that others are willing to show their gratitude in the form of currency.
Check out the New Yorker'sinteractive map which shows how tipping varied during the polar vortex.
[via The New Yorker]