Accidents like this are exactly why I rarely ever use a drive up window.
It is just an awkward position to be in when handing thing from the drive up window to the car; especially to small low profile car.
What is really awkward is when the customer insists on grabbing the cup from you by the lid, for some reason there are people that believe a cup lid has extraordinary gripping power when it’s sole purpose is to keep the contents from splashing out.
Yeah, people do. Were I server, I’d put a holder on the cup and present it so it would be the only good place to take hold of it.