not gonna help on add on items 
here it says $25
Buy Add-on Items
Add-on Items ship with orders that include $25 or more of items shipped by Amazon, excluding gift cards.
When you include Add-on Items with Amazon Prime orders of $25 or more, you'll also receive Prime shipping benefits, including FREE Two-Day Shipping. Add-on Items are also eligible for free shipping. See About Free Shipping by Amazon for more information.
To buy Add-on Items:
Look for the Add-on Item badge while shopping on Amazon.com or the blue title bar around Add to Cart.
Include the Add-on Item in an order that includes $25 or more of items shipped by Amazon.com . You can even create an order containing only Add-on Items that totals to $25 or more.
Note: If you have an Add-on Item with less than $25 of items shipped by Amazon.com in your cart, you can check out with the other items. You can also save your Add-on Item in the Saved for Later section for a future order.
