I pay for people behind me at tim hortons all the time.... My wife does it to!
It's easy... although, If the car behind of me is a minivan full of adults, and the lady has her head out the window for more then 30 seconds I'll just pay my own and move on.
I just look behind me, if its a single person, when you get to the window to pay, he's already ordered and you just say I'll pay for the guy behind me as well please
Get your coffee and drive on...
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_Penelope_