1.
I think COD here means Cash On Delivery. I guess, one should create an invoice on/after delivery. So, if an invoice is not paid in full and payment option is COD, it should mark it as Overdue. Alternatively, you could create a ‘Sales Order’ if the items are not yet been delivered, and convert that sales order to invoice on/after delivery and payment.
2.
Many of the invoice terms can be edited from Options -> Invoice Layout -> Content -> Invoice Text.
Hope this helps.