Easy to do yourself
You can change the oil in the classic prius just as easily as you can any conventional car (my guess is that this is true on the new prius as well). This is one of the easiest things you can do, so you might as well learn to do it yourself. I've never once had a mechanic fill to the proper point when doing an oil change. I've also had problems with mechanics overfilling automatic transmissions, which is very harmful in even short periods of time. The result is, if someone else does it, I check it immediately while I'm on the premises and if (as always) there is a problem I make them correct it immediately.
Also, the above post about synthetic oils in NOT CORRECT. You can still overfill when using synthetic oils. Synthetic oils are, however, better than conventional oils in almost every way; they lubricate better at both higher and lower temperatures, don't break down over time nearly as quickly, and DON'T FOAM as readily as conventional oils. That's probably what the poster meant. It doesn't mean you won't damage an engine by overfilling it with Synthetic oil, there is just a larger margin of safety.