Having gone through something unrelated but ended up in small claims court against a foreign air carrier operating flights out of the USA I can provide you with a few things.

The airlines can be sued in any court in the USA - regardless of where they are operating. Of course if you are AZ you can easily pursue in AZ as Delta operates in the state. As such they will have business address which is half the battle.

The issue though is getting the court to understand why you are owed €600. I faced the same when I sued for a 400% penalty. Fortunately, I had a back up which the court accepted.