Using the Google Docs integration

First, install the DocuPost addon for Google Docs here.

Once you've installed the addon, navigate to any Google Doc.

From the "Add-ons" menu in Google Docs, click "DocuPost" -> "Mail document"

Once your document has been imported to DocuPost, simply fill out the recipient and sender information, and any additional options (such as color printing or certified mail), and click "Send letter"