The Jiyushinkan doesn't have a mat fee. No one gets on our mat unless they're a member or a guest.
If a guest offers a mat fee, we most often decline because we enjoyed the training. However, there is a wooden box on the wall where anyone can drop a "donation, curtesy mat fee, or members' dues" in an envelope. We do not make a big deal out of it. However, it should be good manners to leave an appropriate "gift" even if told that it isn't "necessary."
I'm interested to hear about others' customs.