Is there a difference between a donation and a fee?

The meet-up site for my zen class says $15 per session which seems a bit steep. When I got to the first class, there was an envelope for the money that said “Donation” on it. There seemed to be a number of ten dollar bills in it so I have been putting in ten dollars. The greeter gave me a bit of a hard time about it last week – a new person to me – and now the Web site says $15 more loudly. It also says no-one turned away for lack of funds. Was I wrong to assume the $15 was a suggested donation and that you could choose to donate less?