Upvote:3
This sounds like a "small court" case, but I doubt it'll be worth your time. I'm sure some ToS you clicked agreement to will prevent you from even that, forcing some kind of arbitration, and you'd waste much more money than you're trying to recover just trying to figure out how to pursue that route.
In more general terms, you bought a ticket from a reseller ("travel agency") who has bought a bunch of seats and is operating on an extremely slim margin. Whether or not they ever intended to pass your baggage reservation to the airline, paying you any more than refunding your $20 will probably eliminate all their profits. It is unlikely that you'd be able to get them to compensate you, and since they did refund you the charge - I'm not sure you'd have a case against them in court.
You can file a complaint with a regulator for that industry in the country where they're registered. If enough complaints are filed the regulator may be able to do something about them. It's likely though that they're actually operating in some third country with lax regulations and weak enforcement. Their "contacts" page lists no physical offices or any addresses whatsoever.