This is an Amazon Family exclusive deal, you go to this page to redeem the offer.

Select the eligible device you wish to purchase by clicking the ‘Redeem’ button on this page and add the device to cart. The discount will be automatically applied to the original price and reflected in the order summary at checkout. Complete your order and checkout by 11:59 p.m. (PT) on June 6, 2019.

There are also deals on a 3-pack of the Echo Dots. So if you purchase 3, there are additional discounts that are also applied bringing your total per Echo Dot to $18.32.