I think the problem lies with Baader stating that the filter IS fit for use, and that flat fielding will remove it. So, if Baader claims the filter IS fit and within design parameters, I don't think any bank chargeback is going to have much affect on the outcome. Perhaps they have a caveat or statute of limitations?
I would contact Baader DIRECTLY via
www.baader-planetrium.com, stating your dissatisfaction with the apparent answer you received from them via Teleskop Express representation. Take it from there, and perhaps drop the charge-back bomb.