Today I opened an express mail envelope that contained a requested partial.
Really, you don't have to do this. Express mail is very expensive. If we request a partial, we expect to wait 2-5 days for it, at least. If we for some reason need it faster than that (if you have a book deal with the publisher and are just agent-shopping before you sign on the dotted line), we'll say so, and then you can ask if you can email it.
If you're querying via mail, you're going to be spending money. You can spend it unnecessarily if you want, but don't think dropping a twenty on express mail will impress us. Your manuscript is the thing that needs to impress us. Save some money and send it media mail or at worst, priority. If you live in a state close the agent, totally send it media mail.