You can embed the text you wish to display in the message inside your SQL statement. For example:
SQL: SELECT 'This customer has ' + CAST(count(*) AS NVARCHAR(254)) + ' outstanding invoice/s.' FROM OINV T0 WHERE T0.[CardCode] =$[$4.0.0] and T0.[DocDueDate] < getdate()
By using a UF Macro with MessageBox, you can get multi-line SQL results by using the <NEWLINE> command.