How do I add interest on overdue customer accounts please%3F%0D%0A

 Hello Admin1102, Thanks for posting on the Community page, 

So in Desktop you need to

Go to Lists > Item List

Create a new Item with type Service

Call it something like Finance Charge or Interest

then open the overdue invoice and add it on, you'd need to resend the invoice to the customer so they are aware of the new cost.