1. Barclays will laugh at OP and deny the attempted chargeback. Just wait until there's a discretionary call and Barclays looks at the past record.

2. The merchant vendor is AutoEurope not Hertz and there is no California state court jurisdiction over it. You can argue all you want about this, but it ain't changing.

And, even if you win, you lose as the fact that AE screwed up and forgot the EUR 49 charge will be caught as soon as a grownup there looks at the record, which will happen if you sue.

Just send them the EUR 9 now and save yourself the trouble.