You will not need to pay for the items that you returned. You only need to pay for the item(s) you kept, plus the invoice fee, and if applicable, the shipping fee and the return fee. If you returned your entire order, then you only need to pay the return fee.
Make sure to pay the invoice on time, regardless of whether or not your return has been registered. If the amount for the kept item(s), plus any fees, has not been paid before the due date of your invoice, then any additional reminder fees cannot be removed.