I think you have the right to have the work done at another shop, and I would do so. In fact, when I lived in states requiring inspections, any major flaws would never get done by the inspecting mechanic - I would go somewhere else, for a second opinion and to insure that the flaws were not self-serving. I also used the same mechanic for the annual inspections, when I could.
Check Sears for struts on sale or free labor. Get lifetime guarantees so that you pay labor only for any future replacements. Keep your receipt.
Any muffler shop can generally beat a mechanics's estimate on muffler and exhaust.
If you have never had either of this work done, I would think the mechanic is being honest.