Fedex sends Mahnungen for already paid invoices — What to do?

Hi,

I have recently ordered a few packages that were delivered by Fedex and for which I had to pay import duties (Einfuhrumsatzsteuer). I paid those for each package directly to the driver at delivery, and he gave me a payment receipt each time. Later, I got an invoice from Fedex for each package which states the same costs and that I have to pay 0 Euros (because the import duties were already paid upon delivery), and even the tracking for each package states that I already paid upon delivery. Now I am getting Mahnungen from Fedex for those already paid invoices!

I tried reaching their customer support via phone, and you can unfortunately only resolve billing issues over their website (which the phone automata tells you). They contact you via E-Mail once you make a request on their website. They replied within a day and told me that they could not find the information that I provided them, and that I should forward them a receipt that proves that I paid. I forwarded the receipts that the driver gave me and their invoice where it explicitly is stated that I need to pay 0 Euros, but they ghosted me since that. This was 10 days ago, and I kindly asked them already three times to write me a reply, but without any success.

It seems that this is happening to many other users:

[https://de.trustpilot.com/review/www.fedex.com](https://de.trustpilot.com/review/www.fedex.com)

Does anyone of you know what are my options to resolve this issue with them? I have a feeling that they deliberately designed their customer support to be unreachable by phone for problems like this.

Thanks in advance!