I've used PirateShip for the last 7 years, and has been a great experience, trustworthy, and very smooth to use. I was very hesitant due to its name, as I didn't trust a company that called themselves a pirate : ) but can only praise them, and the price discount is solid! I think I've shipped around 600+ items using PirateShip.
I also get timely emails from pirate ship warning me when USPS is going to raise their prices (sadly that happens too often). They've saved me lots of money. And, I never have to go to the post office anymore, I just print the labels. Then my USPS mailman (who is awesome), picks up my packages at my location, and immediately scans the labels, uploading them to their system to track. This really helps your package move quickly through the system.
One very useful function, saving time, and typo's is the copy & pasting function when shipping using the exact info given to you when creating a shipping label. Also click the price, before pressing purchase to see all the different pricing & service options for shipping that package. I like the option to choose size (%) of shipping label depending if its going to be on a small envelope, or a large box. Also allows me to immediately see tracking number and link to track, which I send to recipient.