Clients who decide how much to pay

How do people feel like they can change the terms of my contract? I have a client who I did about 12 hours of work. I had to also hire a surveyor, and have paid for them out of pocket already in order to get the work done.

I was essentially hired to explain the current drainage pattern, not to fix it and I made that pretty clear when I spoke to the homeowner.

She decided that she would only pay a quarter of what is owed (which give me a whopping $40 over my surveying costs - uh, I don't think so.)

She also went in halves with the neighbor behind her. The neighbor isn't going to pay her half because the letter was addressed to the first woman and, in her opinion, outlines the problems of her neighbor and not hers. Well, the simple situation is that her problem isn't as easily correctable without losing a very large, very old oak tree. Furthermore, she didn't realize that the existing drainage easement took up a good deal of her front yard.

sigh. . .

I'm going to have to demand a retainer up front for smaller projects. They rarely turn out well, take too much of my time, and I usually end up telling the homeowner things he/she does not wish to hear (mainly that the problem is usually costly to fix).