Having spent several years working in law firms I can totally appreciate the need for helping staff better manage snail-mail. I, like they, reveled in the glory of UPS and FedEx's online shipping and tracking tools. If you are still filling out airbills by hand, you should be ashamed. And not to digress too much here, but if you are still doing that and sending packages to your clients who you are TRYING TO IMPRESS, then you should really be embarrased. Do you still have a rotary phone too? Back to topic: there are many companies that allow you to print your own postage (including the U.S. Post Office) but Stamps.com has a legal focus to it. They offer their services starting at $15.99 a month.
As for the USPS - check out their site and services. Their Click-N-Ship service is like taking your computer and all it's information with you right into the post office. Very cool.