Part of the problem is that the standards for generating barcodes have
changed over the years and Word's feature has not kept up. Microsoft has an
article about this here::
http://support.microsoft.com/kb/897290
In other words, if you're using USPS barcodes for bulk mailing discounts,
you have to use a package other than Word.
Although various people have posted saying that that they have never had a
problem when they use a barcode generated from a 9-digit ZIP (rather than
from a street address+ZIP), the USPS also has an article saying that
"unless you are preparing a mailing for automation rates, you should NOT put
barcodes on the mailpieces. As long as the mailing address is correct, your
mail will still be processed efficiently without a barcode-and
you won't need to worry about having the wrong barcode on your mail."
http://www.usps.com/businessmail101/misc/discountsBarcoded.htm
Like Graham, as I don't live in the U.S. I have no idea whether inclusion of
the barcode speeds anything up, but I suppose I would have to take account
of what the Postal Service itself says.
If you still want to use them, you can still insert them manually using
ctrl-F9 to insert the special field code braces and typing the word BARCODE.
The content of the rest of the field would be identical to the ones you used
in Word 2003, so if you have a document created in Word 2003, you should be
able to cut/paste the field codes into Word 2007.