Have a non-printing notes area on invoices/quotes/orders for recording details, reminders, explanations, even cautions about the transaction.
Many transactions are a little out fo the ordinary in some way and these details are often lost or forgotten.
Five minutes recording salient details at the time can save an hour of trying to reconstruct things a few week later.
Even better than an area on-screen woudl be a tab or link to a full screen notes area linked to that document.
A full screen wodl allow multiple dated notes.
Our precios program WorkFlowMax has this and it was immensely useful (of course that program is very limited in other respects).