Found it folks... thought I'd update how if that helps anyone.
In the relationship I created between the 'Customer' table and the 'Invoices' table, I set the records on the 'Invoicing' table to sort Descending. That way, the most recent entry is displayed.
Can't believe I didn't think of it earlier... Doh!!
Ad another portal into Invoices to your layout, make it 1 row tall, sort it by datePaid, descending, and put datePaid into the portal.
I'm assuming I can use Conditional Formatting but a unable to find a formula.
CF would be useful to flag the record with the most recent date in a list; but it won't help you to isolate the record with that specific value.