If you've lost the device due to failure of the mount (as opposed to not clipping it in properly), I'd absolutely be persuing Garmin, it's their fault.
OTOH, if you've just been a spoon and not fitted it properly, then you're just going to have to suck it up I'm afraid.
Sucks, mind. I lost 80 quid's worth of bike computer after leaving the house in a hurry to catch a train; for some dull reason I'd left it floating loose in its mount rather than properly snapped in when I put the bike away previously, I guess I'd meant to take it off for storage and forgotten. Ten yards from the house it somewhat obviously parted company with the bike – I heard it clatter to the floor, couldn't immediately see anything I'd dropped and didn't know what it was so carried on pedalling.
I found it later when I got home, but sadly so had several car tyres by that point. I couldn't even salvage the battery, it was that badly mangled.