Despite downloading the Add-in for Microsoft Word Home and Business 2016 to my Windows 10 64-bit machine it would not install. I could see the Add-in under File - Options - COM Add-ins, but despite trying to click the box to the left of it, it would not install.
After calling Stamps.com I found that it does not support 64-bit MS Word, it only supports the 32-bit version.
So to save you frustration I'm posting this to warn you.
A workaround is to print your envelopes using Mail Merge from MS Word without the stamps and then print them with stamps only from the online version of Stamps.com.
Here's more info - lest you try to run 32-bit addins on a 64-bit installation.
I haven't tried Endicia? Are there other solutions?