When customising the Customer Statement, Statement Table section, please include the option to add the Invoice's (Retainer too!) Reference field into the Transaction Details column.

For example, the column currently says "RI-2018-00004". If the Invoice's Reference field has "Bulk Widgets XYZ for Project ABC", then the Customer Statement's Transaction Details column should be "RI-2018-00004: Bulk Widgets XYZ for Project ABC".