It seems that sending this with every XML request is the only way to use the UPS Tracking API:
<?xml version="1.0" ?>
<AccessRequest xml:lang='en-US'> <AccessLicenseNumber>
YOURACCESSLICENSENUMBER
</AccessLicenseNumber> <UserId>
YOURUSERID
</UserId> <Password>
YOURPASSWORD
</Password> </AccessRequest>
So basically you can't use it in a desktop app, because your users would be able to find your password. And yes, I could run a backend on my own server, but that's needlessly complicated. Does UPS provide a sane way to authenticate without having to put a copy of the password in each copy of my app?