Do you charge interest for late payment when invoices are past due date?

We demand interest for late payment after having sent a third reminder to the debtor 25 days after due date. However, these are not always charged in reality:

• Sometimes they are mentioned in negotiations with a debtor to finally receive payment in an amicable manner. This is the preferred way over a legal procedure which is time-consuming and at the same time not always guarantees success.
• As the costs of recovering the interests are often higher than the interest itself, legal collection of interests is often not economically justified in terms of costs.