Similar to Payment Instractions there should be a field where you can enter some description about the invoice itself. Being Projectoriented this description field would kinda summarize the conditions for the invoice project itself.
For Example a text like:
“Invoice for the Wedding at 23.10.2015 and will be held at whatever church. In our Pre-Talk in the Coffee-House you wish for this and that kind of photos. The desired moments are Kissing, Exchange of Rings, Family reactions etc. bla bla bla.”
The Placement could be in between Names/Adresses and the Positionlist.