the click handler on $('#save_box_button')
gets attached every time you call customize_box which is every time $('#customize_link')
is clicked.
instead of $('#save_box_button').click(function(){
you should use
$('#save_box_button').one("click", function() { ...
that way, the click handler only gets attached once.
A second option would be to remove the possibly present click handler before you reattach it:
$('#save_box_button').off("click.savehandler").on("click.savehandler", function() { ...
notice the domain "savehandler". it prevents other click events from being detached by the off
-method which i think is a good practice using this method.