To the best of my knowledge there is no standard rate, it should be reasonable. I don't use this feature but if I were to use it I would consider 1 to 1.5% per month on the outstanding invoice over sixty days. If you are going to charge interest for over due invoices this action with the interest rate to be charged must be clearly stated on the original invoice at time of issue. These are just my personal thoughts.

Thank you Alwyn