I am having a prolbem with adding interstto my overdue accounts. I have an aging field and an orde total, but is there an easy way to automatically add interest to an invoie that is out 30 days and then keep updating the interst with each statement?

It's easy to add interest. The trick is figuring out when not to add interest. It depends how your database is set up. Here are a couple of ideas...

If you are tracking the payment of individual invoices by applying payments to the oldest invoice and marking it paid (by setting a paid field), then use the paid field and the invoice date as a test to determine whether to accrue interest.

If you are only keeping a customer balance owing, you will need a new charges field, and an existing balance field. Every month, add interest to the existing balance, then add the current charges to existing balance. Clear the current charges and calculate the new current charges.