No, there is no minimum amount and they are not allowed to accept tips so don't bother.
Schwan's is still a family owned company based in SW MN started by Marvin Schwan who's parents owned a local dairy. He found that the farmers in the area never bought ice cream because it would melt on the way home so he figured out a way to bring the ice cream to them and turned the company into a multi-BILLION dollar international company. His children still own the company today and it is still headquartered in the small MN town where he grew up. It is a great American success story. Marvin was a very, very generous man and never wanted recognition for what he did.