Look at Andrew's last (modified) response suggesting use of a StyleRef Field.

Thank you Andrew and Charles,

I guess I am exasperated because it's 35 different clients and it seems that even doing it your way, I have to "add envelope to document" for every single one.

Can you offer any other specific instructions for using Style REF? The merged letter looks like this for the 32 recipients:

January 6, 2020

Mr. John Smith
1234 Main St
Hollywood, CA
90210

Dear Mr. Smith:

Blah Blah customized body.


Best regards,
Jane Doe

It seems if people are printing a merged letters at once they would want to print ALL the envelopes at once. Is it worth sending this feedback to Microsoft? lol How is this not a thing?