Amazon Partnered Carrier Program: Additional UPS Service Fee for shipping Europe to US

We use Amazon Partnered Carrier Program (UPS) to ship our goods (FBA) from Germany to US. Now we receive a separate invoice with an additional service fee from UPS. The reason is, that there are no digital documents (mainly invoice) . However, in the sellercentral shipment process there is no option to upload digital documents and UPS could not give us a process to hand-over the digital documents. When we explained our problem to the seller upport we got a general explanation for the program :-) Is there any solution for that specific problem out there?