A suggested overview to help with planning the Invoice layout:
Create a field 'payment' (number) add this to the bottom of the invoice layout and make it part of the calculation that totals the Invoice. So it would be item 1 + Item 2 + etc - payment = Invoice total. When you enter the amount paid it will take it off the Invoice total. If fully paid the Invoice =0
You can also create a paid field (text) something like If (Invoice Total = 0; "PAID";;) This would print PAID across the Invoice. You will need to look up this in the Manage Data Base/Fields.
The solution starters are fine, however I found that fm starting point has a bit more functionality, including paid dates/shipping dates etc on the invoice.
It also includes a deposit field, which is subtracted from the invoice total, this could be simply relabled.