For a nice label printed out with FREE delivery confirmation, use this web site from USPS
Here
Click on the dot that says
Create an online label for shipping a package
Then scroll down and fill in the required info below and click submit. If you don't have the Zip +4, don't worry, they system will check and search it out for you, I then highlight and copy the Delivery Confirmation number from the resulting label on screen, near the bottom and paste that into an email to the buyer.
You can buy full sheet label paper or just print on regular paper and tape over it onto the package.
I use this all the time, for a couple of reasons;
It provides a legible label so there shouldn't be a mixup in delivery
It checks/verifies and provides you the zip +4 for both your address as well
as the person you are sending to, several times I've ran into where the address showed up as not good, going back to the buyer it turns out he had transposed something, so that is a great catch to prevent lost packages.
and of course, you get FREE delivery confirmation, as noted in the above posts, it doesn't always get you much, but if everyone is on their toes, you'll have an estimated time when it arrived to there house. If they don't scan it on arrival, there's nothing to be helped with that, but my post office says they are monitored to be sure they do the scanning thingy.