The package is used for sending items through a Mail Box . It holds up to 6 items (see Envelope for less storage). Right-click with it in your hand to open up the GUI where you can put in your items. Once you have finished, press the "Sign" button. This will seal the package and attach your name to it. To put a package into a Mail Box, hold shift and Right-click on the Mail Box. However Packages cant be placed in the world.