In Manage | Database | Relationhips, you can find this relationship and double click the line to bring up a dialog where you can specify details. One of the options in this dialog can be used to specify a sort order for the related Invoice records. If you specify a descending sort order on the sale date field, then your calculation can refer directly to the Invoices::sale date field and it will refer to the invoice with the most recent sale date for that cusotmer.
Do look before you leap here. This could change the appearance of a portal or the results from a script that uses this same relationship. You may need to add a new occurrence of Invoices so that you can specify a sort order with it and leave your existing relationship untouched.
Thank you, that did it.