Depends on how you want to calculate the balance due.
If this is the balance due for all invoices for a given customer, put it in Customer.
If this is a balance due for a given invoice, put it in Invoice.
So basically I gather that you can do all sorts of self-join and create portals to receive different types of data.
Is their any limits to a number of self-joins a table can have?
Eventually, you can run out of space to place additional Table occurrence boxes, but you have to work at it. Keep in mind that you also have summary fields which can compute multiple record totals without using a relationship at all.
Thats what I did is us a summary field for my balance due.
My thoughts on the table occurrences was not related to the balance due field....just in general for other reasons...
So basically, you can create table occurences as many times as you like until you run out of space(not saying that i have that much of a need to do that).
I was just getting some idea on table occurrences...before i started creating them...don't want to jack anything up.