MailMerge using Office 2007 Word and Access

I have a client that needs a mailmerge that:

sends a letter to each of their clients that have past due invoices. The problem I’m having is that for clients that have more than one invoice past due, the mail merge prints a separate letter for each invoice instead of including all the past due invoices on one letter.

I’ve googled, but no joy found.

Any suggestions would be appreciated.