Trick question. 


When Amy makes a purchase from Ben, Amy would have sent a
Purchase Order to Ben.
When goods are delivered to Amy, Amy would have to check
the Delivery Note to her Purchase Order.
After signing the Delivery Note, Ben will send an Invoice to
Amy.
Credit Note is for goods returned, overcharged invoices,
etc.
I just learnt it from Google. 
You can search "accounting reference desktop accounts
payable transaction flow", click on the first search
result book by Steven M. Bragg, Chapter 17-3, page 194.