Nifty:
As we've all heard before, "it takes all kinds." It's a pity this person gets to deal with the public, but if the shop owner is also an ___ ____, why would she (the clerk) act any differently?
I feel for your embarassment, but it is the clerk who should be embarrassed. I can assure you those who heard the exchange were just glad that it wasn't them. They thought no less of "you for making a mistake any one could have made.
Don't let this stop you from attending your meeting. I've always told my kids, "you can go anywhere if you act like you know what you're doing." Walk in there like you own the shop and have a ball. I'd likely make some joke (out loud) about being yelled at by "the teacher". She'll get the message.
There are stupid people everywhere. When I encounter one in business or on a personal level. I just wash my hands of the event, not the person.