This is why no one should ever accept a charity lecture or brief local appearance where a fee is involved, at least in England. The sum is invariably tiny, but the organisers will most definitely feel, once they have pressed a few coins into your hand, that they own you body and soul. If you must do these things, and sometimes one must, then please do them for nothing. Do them out of the goodness of your heart. The money will make no difference to your life, but you will never have to endure the sense of being a purchased hireling, since you retain the whip hand of your generosity. Better yet, donate the fee you might have had to their cause, or to something equally worthy, and add a halo to your head for good measure.