Is the “Trinkgeld” field stored as a fee in the order details?
If so, with the Premium Templates extension, you will get access to the customizer, which allows you to remove specific rows from the totals’ area, like the fees row:
If you have PHP skills, another way to achieve it is to unset this specific row from the totals’ area using the wpo_wcpdf_woocommerce_totals filter hook (source). You can see a use example in my reply here.
