I have my own business and was taught something interesting which I have used a lot. Say "no" with money.

Your reply is: "m'am, our hourly rate is (fill in blank). By the looks of what is needed, I will need at least 2 hours with your bike".

By doing this, you haven't declined the work, you have set a basis for working on her POS. If she wants to pay it fine, if not so be it. Remember businesses are in business to make a profit, not do people favors.

Also post a sign with set minimum prices for frequent types of repairs. Your customers can sit and look at it before they even try and ask you to do anything.