One option would be to use a DATABASE field in a normal ‘letter’ mailmerge main document and a macro to drive the process. An outline of this approach can be found at:
Many to one email merge using tables - Microsoft Community
Conversely, if you're using a relational database or, Excel workbook with a separate table with just a single instance of each of the grouping criteria, a DATABASE field in a normal ‘letter’ mailmerge main document could be used without the need for a macro. An outline of this approach can be found at:
Mail Merge - To a Word Table on a Single Page - Microsoft Community
For some working examples, see:
https://www.msofficeforums.com/mail-...-multiple.html
https://www.msofficeforums.com/mail-...tml#post151706
Merge excel list into Word Receipt
(the second of these uses a macro to apply some additional formatting).

The DATABASE field can even be used without recourse to a mailmerge. An example of such usage can be found at: https://www.msofficeforums.com/mail-...html#post67097

__________________
Cheers,
Paul Edstein
[Fmr MS MVP - Word]