They had a $100 estimate fee and I assumed I'd get an estimate with a diagnosis. After one month, the shop doesn't know what's wrong but says I lose the $100 if I want it back. I asked if they might know in 2 more weeks and they said probably not.
Rff000
If I were you I would not threaten them with any action until you have the amp back in your possession and off their property. Hopefully you have some type of a receipt that shows you paid them $100 to look at the amp and they would then follow-up with what was wrong with amp and what it would cost to repair it.
If so, and you paid the $100 using a credit card, then you can do a chargeback. If not I suggest you lick your wounds and move on, and be thankful you got the amp back, hopefully in the same condition you took it to them and not in a cardboard box in pieces.
.