Raising prices at shows all depends on how high they are raised.

If you have to pack your merchandise, drive several hundred miles, stay a few nights in a hotel and pay a vendor fee of $500 (or more) then adding 10%+- is reasonable.

If I can pick up an item at an event and pay the same as it would have cost me online PLUS shipping, I am a happy camper because I got to see it before buying and it is going home with me.

As to being a captive audience...most shows have several kilt vendors, and t-shirt vendors, and food vendors...