This looks like a lot but it is essentially a one-time setup.
First, create a "dummy envelope main document"...
Tools > Mail Merge > Create > Envelopes > Active Window
Get Data > Open Data Source
[Browse to the desired data file]
Open > Set Up Main Document > OK
Insert Merge Field (for each field), eg.,
Name
Address
City, State ZipCode
Then, click the Insert Postal Bar Code button
[Choose the field containing the zip codes]
[Choose the field containing the street addresses]
OK > OK > Close
It should look something like:
{·BARCODE·"{·MERGEFIELD·Address·}CR
{·MERGEFIELD·ZipCode·}CR
"\u·}CR
where CR a paragraph marker/Enter.
Now, click anywhere in the address block
Double-click the gray frame
Click Remove Frame
Shift+F4 until the block is at the left margin
Alt+F9 (to reveal codes)
Click in the left margin beside {·BARCODE ...} (to select)
Tools > AutoCorrect
Type: \barcode
Add > OK
Now, anytime you want to insert a barcode, place your cursor anywhere in a merge document and type \barcode and press the spacebar.
Of course, you would need to be consistent in naming the zip code and address fields each time you set up a new merge document; otherwise, you will need to edit the field names after inserting the block.