Prime only.
Use promo code DASHBDAY to get up to three Dash Buttons for 99¢ each. According to Android Central you still get the $4.99 credit on your first use. The link below says you get $4.99 credit when you buy for $4.99, but since you're using a promo code I would think you still get the full credit.
https://smile.amazon.com/b/?ie=UTF8&node=10667898011