The following message from February 2006 may help:

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I have read through this group and found this question several times, (that
word does not recognize stamps.com software) and the answers are always to
check with stamps.com help center.
I did, and would like to post their answer, as it took me a couple days to
get an answer back from them. I hope it helps somebody!
"To enable the plug-in for Microsoft Word:
1. Make sure Word is closed.
2. Log in to Stamps.com, and click on Help at the top of your Stamps.com
software.
3. Click on Download Plug-ins, and choose the Plug-ins tab at the top of the
next screen.
4. Place a check in the box next to your version of Microsoft Word, then
click Next.
The software will install the plug-in and may ask you to restart Stamps.com.
You should now see a Stamps.com icon in your Word toolbar.
This link provides step-by-step instructions on how to use Stamps.com with
Microsoft Office:
http://www.stamps.com/tools/microsoft/
There you will see exactly what to do, along with pictures of what the
software screens look like while performing these tasks.
Regards,
Customer Care Specialist
Stamps.com Internet Postage"