See this, it is a rough draft, wasn't finished at the time, was showing a popup edit:
https://drive.google.com/file/d/0B1_PFw--3o74TC16eXRBYXZBNFk/view
So there I am working with an invoice. It happens to be for spays and neuters, So whether an event, a service, whatever, look at it like a master detail. In my case I went by invoice number, where of course the customer id is on the parent invoice. (In work here, wasn't final), all fields not shown in parent.
It's the invoice id that relates the parent and child, you do not need a customer on the "line items".
https://i.imgur.com/7l7DaPM.jpg

- Parent = the invoice
- child = the spays or neuters in this case
Image, top is parent, under is children (line items).
This was also for a non profit. And don't over think this stuff, keep it simple.