Ask to talk to the “other vendors” who are doing this for free. If they are, then maybe it is a good marketing opportunity. I know that a lot of harpists play at those weekend wedding shows for free, while all of the other vendors have to pay fees for their booth. If you do play for free, make sure they will let you hand out brochures or cards. As a marketing strategy, you might contact other wedding vendors in your area(florists, caterers, and in particular Justices of the Peace and ministers, etc.) and see if you can get them to include you as a part of their offering. I know a harpist who for years was hooked up with a very pushy Justice of the Peace whose brochures(the Justice’s) offered his services with the harpist. She did many many weddings because of this, often 5 or 6 weddings on a weekend.