I hate to deal with companies/individuals who's opinions are so fossilized that they can only see their version of the truth. - It sounds like there was a mistake made on the UPS end of things where they incorrectly invoiced for a different package that wasn't yours and Reverb just took their reporting at face value and back-charged you. Not right, but looks like that's what happened...
Everything everyone has said about the BBB is true. I believe the folks on TB who say the were able to "buy" a good rating, although that does make me sad. BBB does not investigate. BBB provides a publicly accessible rating that is supposed to be an accurate barometer of whether or not a business is providing what they say they will provide in a way that is pleasing to it's customers. The worst you can do to someone there is ding their rating a little...
Sadly, the rabbit hole of pursuing an option through law enforcement would be tedious, frustrating, and take forever to make you whole again. They will give up on it long before you will. Unless you want to go for a lawsuit against Reverb. The result would be roughly the same, expensive, and have no guaranteed results.
This is your best bet:
Another option is to dispute the charge with your bank, which may force Reverb to provide support for the charge
If you purchase with certain credit cards, you can dispute anything, get your money back practically immediately, and then the credit card company goes after (in this case) Reverb for the $20. It isn't justice, it isn't poetic, it isn't right, but it is the most efficient means to an end. I've been lucky. I haven't had to dispute anything in a very long time, but I have done it before. Once with Discover Card, and once with my Chase Manhattan Mastercard. Both went exactly as I said. I don't even know if either company was able to get their/my money back from the misbehaving retailers, but I was credited and never heard of the issue again. Maybe the credit card company wrote it off, or maybe they have scarier means of showing the light to retailers that misbehave. I prefer to believe the latter because right is right. No one on either side should be harmed in a transaction.