Addons are available periodically when there are brand updates.

Via the FAQ

"Q: What are Addons? / Can I add on other full sized items?
A1: Yes! By all means! You are welcome to add any in stock item during initial pre-order from whatever manufacturer we happen to be currently running a pre-order for. There will be a brand specific “Addons” listing seperate from new release listings. 

A2: As mentioned above, Addons are additions to our order.  They are most useful for internationals and folks who want to cut down on their shipping for financial or environmental reasons."