I'm going to shovel snowy driveway and deposit it on my roommate's car
why do you want to deposit snow on your roommate's car?
Because he made me do all the shoveling
If you think your roommate is not doing their part,  you should ask him politely. Depositing shovelled snow on their car will unnecessarily lead to a fight between you guys.
He should've shoveled the snow himself.
I understand so the next thing you need to do is talk to him.