For quite awhile now, the touchscreen on my Zumo 450 has been acting up, such as not recognizing a touch, recording a touch at a different location on the screen, and appearing stuck after a touch. Recalibrating helped only some times. Garmin no longer supports the 450 so sending it to them was out of the question. I believe they still support the 550, however.
So looking for alternatives online, I found several sources for replacement touchscreens and also instructions on how to replace the screen. I bought a replacement screen for less than $20 and finally replaced it yesterday. It was a pretty straightforward process and it works great now. So if your Zumo 450/550 screen is acting up, there's hope yet! You can do it yourself for $20, or send the unit to a 3rd party repair shop for about $40.