Unable to fix their own error - but still holds customer accountable

Bought a product from a website in the US that was ~R1,500 in value. Got hit with an import tax bill from FedEx of ~R4,500. Multiple calls and emails to their call center to get it sorted but to no avail. The correct amount is supposed to be ~R420. I've told them multiple times that I`m willing to pay the R420 right there and then if they amend the amount on their side.

Today I received the following email from them: "My apologies for the delay response, unfortunately we have not received any positive feedback from our VOC team regarding your query and we have to follow FedEx processes

Please see attached Final demand letter"

This is basically telling me that FedEx can't correct their own errors in their processes and the customer has to suffer as a result of it. I've phoned them again and was told to pay the R4500 and then they will give me a refund later...given that this has been going on for ~3 months, I`m very reluctant to give them money and then hope they will refund the difference after they are able to correct their own error somewhere in the future.