Should I go through with this or back out?

My GF told me about two weeks ago that a friend of her's from work, who I don't know, is hosting a bachlorette party for one of their co-workers at her home next week. She was looking to higher a guy to tend bar and serve drinks to the ladies and was willing to pay up to $300 for about five hours work. The one catch was I was to tend bar wearing only a pair of biker shorts. I agreed but now this weekend we went to the house where the party will be to discuss some details. During the discussion the hostess handed me a box and told me to go change into my "uniform" so she can be sure she likes it. To my surprise the box contained a microscopic g-string. I told her I was not going to wear that for the evening but my GF told me it would be fine. Finally the hostess offered to pay me $500 for the night if I wore the g-string and I said I would think about it. On the way home my GF kept telling me to do it as we could use the money which is true but I don't know if I should. Thoughts?