The payment method via invoice is no longer available, but there are still users who have the older plans and pay via invoice.
The invoice surcharges are generated if you have chosen payment via invoice.
These transaction costs are aggregated to every new invoice and are lapsed if you pay via credit-card, SEPA Direct Debit or PayPal.
The transaction costs are indicated in the pop-up-window (Before payment is done).
Learn how to change your payment method here
Removing the invoice surcharge from an already created invoice is not possible.