If you have Endicia integrated in your admin, you can print shipping labels directly from the Orders page. When you expand an order, there is a "Shipping Label" section below the line items of the order where you will print labels.
To print a label, make sure the weight and mail class are both correct. The weight should be filled in by default based on packaging presets and the weight of the items in the cart (for more on packaging presets, go here.
As of 7/9/2014, some new shipping options have been added to the Endicia integration. Here is a breakdown of some of the more commonly used shipping options:
"First, Flat": Large Envelope. This label prints out horizontally, so make sure to change the orientation from portrait to landscape in your printer settings when printing this kind of label
"First, Letter": The cheapest shipping method. This label also prints out horizontally, so make sure to use the landscape orientation on it as well. This is used when shipping with a basic envelope
"First, Parcel": Use this kind of label when shipping a bubble mailer or other kind of package 1st class. USPS classifies bubble mailers as packages, not letters.
"Priority, Flat" and "Priority, Parcel" both cost the same. One just refers to a letter while the other refers to a package.
"Priority, FlatRateBox" is the medium flat rate box
There are now three Express options that were not available before: Flat, Letter, and Parcel. All of which cost the same, but just refer to different packaging formats.
For more information about label printers and printer settings, go here.