What happens when I don't want to sell the invoices? (for supplier)

You have no obligation to sell the invoices your debtors have approved. You can only sell the invoices based on your current need. The choice is 100% yours. The cost rate is only applied when you decide to sell the invoices and only on those invoices. 

Invoices you don't want to sell will be paid by your debtor on invoice due date. We will transfer the money to you as soon as we have received the payment from the debtor and in this case, no cost rate is being applied. 

So to conclude - We are applying cost rate only on sold invoices.