How does interest accrue in collection and at what percentage rate?

Interest on arrears begins to accrue from the due date if the invoice if the invoice has not been paid by then. If a due date is not separately specified, interest on arrears begins to accrue 30 days after the invoice has been sent or a demand has been made for its payment. 

When an invoice comes to us for collection, interest on arrears accrues for as long as collection continues. The rate of interest on arrears is 7 percentage points higher than the reference interest rate set by the European Central Bank. In commercial agreements, i.e. in business invoices, the rate of interest on arrears, however, is at least 8 percentage points higher than the reference interest rate set by the European Central Bank. Please note that the interest rate may be higher than this as a result of the original agreement. You can find the interest rate applied in your case in the letter you received.