I'm going to call people and demand they pay their fees
Do the people already know that they are required to pay their fees?
Yes, I told them about that little detail
It's acceptable to let people know that they need to pay fees that they owe, as long as you are polite about it. Sometimes people forget things, so calling them to remind them is a reasonable thing to do.
Pfft, like if you are paying me back.
Well if the people owe their fees than it makes sense that you are going to call them. Try to remind them in the most polite way you can just to give them a reminder. Also, letting them know the consequences in a way that is not condescending is helpful as well.