Fed Ex, mail, scan and email. The odds of somebody snooping your mail are infinitesimally low. Unless you gave somebody else the password. Or you are a high-profile criminal. Or you are sending it physically to an office, where mail is routinely opened by clerks/secretaries. And if they work with tax documents, they are unlikely to be unprofessional enough to actually read the contents.
Or you could allow the other party to connect to your computer via Remote Desktop or VPN and let them download it.
A Windows 7 or Vista VPN is actually really simple and quick to set up.
http://www.publicvpn.com/support/Vista.php
If you have a web site, call the other person with a link, post the document, delete it as soon as they have downloaded.