From Webkinz Insider Wiki
Kinz Post is the section of Webkinz World where you can send gifts or notes to people who are on your KinzChat phone friends list. It does cost KinzCash and the specific cost is dependent on how many items you are sending,what wrapping paper you use,what paper you write on and if you use stickers.
You can find KinzPost in the Kinzville Map. Once there, you can choose if you want to send a note or send a gift.
Each item costs 15 KinzCash to send. You can send a maximum of three gifts. A player can only send one gift to each friend per day.
In June 2011, it became possible to add Webkinz Host Friends in Webkinz World and to send packages and letters to Webkinz World characters. These hosts occasionally send back to players a response letter and more rarely, a gift.