Can I be charged a late fee/be taken to small claims court on a bill with no due date?

You've had a sweet deal with this company and it seems to be over. I'm surprised they kept filling your tank and waiting for their money until they filled the tank again. When you go to the grocery store you can't tell them that you'll pay for the groceries the next time you come in for more groceries. The little card is a bill if it states how much is owed and you need to pay it as soon as possible.