Many to One mail merge is difficult to achieve, however take a look at Paul Edstein's tutorial at
http://www.gmayor.com/Zips/Catalogue%20Mailmerge.zip. The examples use Excel data, but the principles are similar whatever the data.
As you noted, third party add-ins like my own, work with Excel data, and that was complicated enough without having to cater for Access data, though I may investigate it at some future date. In the meantime, the simplest solution might be to export the data you require to Excel and then use
http://www.gmayor.com/ManyToOne.htm