You can send packages of tack or horses through the post office.
Click on the Post Office icon and then click on the icon.
Type the name of the player you wish to send the package to. Select the items or horses to be sent and use the arrow keys to move them then click the 'send tack' or 'send horses' option depending on what you are sending.
Note: You can not send items, materials, feed, or treats as a package.