how to print a RETURN ADDRESS on Office 10 envelopes on iMac?

Please review https://support.apple.com/en-nz/guide/contacts/adrbab40ca1d/mac

I have been using EasyEnvelopes (available in the Mac App Store) for years. It allows you create a custome return address, including graphics if you wish. It also includes the postal barcode options for the main address. Of course, it supports almost all types of envelopes, including the ability to customize a particular sized envelope.

I don’t believe the Contacts app supports return address printing.