You need a field to uniquely identify each customer, ideally this is NOT the customer's name as names change from time to time. Let's assume that you've set up a serial number field named CustomerID.
Your layout refers to a Table in Layout Setup... We'll call it Invoices for this example.
Make this relationship:
Invoices::CustomerID = Invoices2::CustomerID
To make it, find the table's TableOccurrence box in Manage Relationships | Graph and drag from customerID outside the box and then back to the same field. A dialog box will pop up so you can create Invoices2, a table occurrence that points to the same table as Invoices. This is called a "self" relationship.
DoubleClick the relationship line and click the sort check box on the Invoices2 side in the dialog box that pops up. Specify that the records sort by your Payment date field in Descending order.
Now you can place the Payment date field from Invoices2 on your layout and you'll see the most recent payment date for the current customer.
You'll have to look over your tables and relationships carefully to see if this works as I've posted or if we'll need to make changes. You might have your payments listed in a related table linked to your invoice record, for example, and that would change the details some, though the technique still can be made to work.
You can a one line portal that is sorted descending on payment date. Put that on the layout.
Thanks PhilModJunk and mr_vodka. I created the self relationship, but ended up using the unique field Contact ID instead of Payment Date and then created the one line portal with the date in descending order. It appears to be displaying as I hoped. Thanks again.