The simplest option would be to use http://www.gmayor.com/Envelope_Label_Add_In.htm .

See also http://www.gmayor.com/changing_envelope_layout.htm which demonstrates how to configure the envelope tool to format the address.

Mail merge is overkill for a single envelope, but if you want to use that, then don't use the addressblock field. Use instead the individual fields that make up the address.

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Graham Mayor - MS MVP (Word) (2002-2019)
Visit my web site for more programming tips and ready made processes www.gmayor.com