If you have an paypal account, you can use that account to easily purchase an online shipping label for shipping any packages domestically.
Paypal has the option to use USPS or UPS. It is fairly easy and efficient to use but this service is hidden and hard to find.
Below is the link you can check out.
I have a paypal account and I have used it a couple time. It definitely beats personally mailing it at the post office.
Here is the screenshot of what the service looks like.
For more informational details on this service, please check out this link: http://www.ebay.com/gds/Secret-PayPal-Shipping-Labels-Stamps-off-eBay-Mailing-/10000000007215096/g.html