This is how I would do it:

1. Create a new document.
2. Format it as an envelope under page layout.
3. Insert two borderless frames or text boxes, one for the return address and one for your delivery address.
4. Type the primary address into the second text box.
5. Under the Mailings Tab, Start Mail Merge, pick letters.
6. Tell Word to use your current document as the main document.
7. Attach your data source.
8. Insert merge fields for your return address.
9. Save your primary merge document.
10. Try finishing the merge to edit documents so you can check how it looks.
11. Finish the merge to your printer or print the document you just created.

Mail Merge

Word Mailmerge Tips & Tricks | Windows Secrets Lounge