I tried something that worked for me. It was the opposite (couldn’t get a response from a customer).
Write a nice note and print it, free from animosity, grief, etc. Keep it positive. I tried to make mine light, and humorous. No hard feelings. Just want to get your amp back.
Hand deliver it to the known address and tape/attach it to the door in a waterproof packing (zip lock bag, etc). Include a self addressed, stamped envelop containg a form which would include, for example, a time you can meet up with the servicer to retrieve your amp. Leave a blank for an "other" option to be filled in by the servicer. He might be embarrassed/frustrated, doens’t want to come face-to-face with you and may prefer to get a 3rd party involved. Drop if off at a nearby diner/bar/cannabus outlet, for example?
This would represent a good faith effort to take responsibility for the situation and, hopefully, get your amp back.
It that doesnt’ work, see if their is an industrial equipment rental business close to the destination, and take out the front of his building. That should get his attention?