Just saw this promotion today.
It is decent deal. $19.95 for two (less than $10 each)
This one also works with Apple HomeKit.
Here is how to get one free
1. Add all products described in the promotion to your shopping cart, including the one that's free. (so you need to add at least two in your cart)
2. The value of the free item will be reflected on the final order checkout page and discounted proportionally from the price of all promotion items in the shopping cart.