You start by making a transactions under the invoice of the invoice payment.
2. then you write the amount of only the invoice payment.
3. Under bank account, you choose, the payment bank account.
4. Under notes you write a text, like the date or invoice nr. remember to choose the invoice you have to match it with, and the date of the invoice payment.
5. Now you will see that there are 20 kr. left on the invoice, we assume is the fee on this payment. So we have to make a new transactions.
6. first you write the 20 kr. under amount as we know thats the fee, and then choose bank fee under bank account, if you don't have this account, you have to create it under bank account. remember to make a text like foreign fee, also choose the correct invoice, and the date of the payment.
7. Now your invoice are posted, with the payment, and the fee correctly.