This says that romotional credit expires at 11:59 p.m. (PT) June 18, 2018. That's pretty soon, so I wonder if that's the last day you can register. The Dash Wand is $20.
https://www.amazon.com/b?node=16962180011
For a limited time, receive a $20 promotional credit when you register your Amazon Dash Wand with your Amazon Prime account.
This is a limited time offer.Offer only to Prime members who register an Amazon Dash Wand with their Amazon Prime account.Offer good while supplies last.Offer limited to one per customer and account.Offer may not be combined with other offers.The maximum benefit you may receive from this offer is $20.00.The $20 credit will be added to your customer account when you check out with an eligible item.Promotional credit expires at 11:59 p.m. (PT) June 18, 2018.