Daniel; I'm reading from the 2007 Toyota maintanance and operation manual (the world's most un-read best seller), and for your Matrix (similar to a Corolla) we have the following at 30,000 miles:
- Change cabin air filter
- Oil and filter change
- At 40,000 miles INSPECT fuel lines & connections, and and gas cap.
- If operating in very dusty or other adverse climate or off road conditions, change (not FLUSH) the manual gear box & differential fluids every 20,000 miles. This does not apply for normal driving.
- Other normal checks and replacements are manual transmission fluid and transfer case fluid. at 40,000 miles for AWD and 4wd only, no required service for 2 wheel drive Matrix!
- NO FUEL INJECTION SERVICE is specified; today's gasolines do not require this, since there is enough detergent in them already. You can pout your own $9 bottle of flush in the gas tank if you so desire.
So your "trusted mechanic" really has NO CLUE as to what your Matrix really requires, and sells whatever service he has a machine for. In this case, fuel injection and automatic transmission flush!
Please read your owner's amnual, copy out the relevant pages and show these to the service guy.
You will likely get a similar pitch from the Toyota dealer, but they would not want to flush a manual transmission.