Several dpreview forum members are reporting that they were able to get 20% cash back when purchasing the Pentax K-1 from Amazon US with the Amazon credit card (see this and this post). I could not confirm this offer because I already have the Amazon credit card and the offer did not show in my account.
Update – here are the details of this offer:
Now through December 22nd, eligible Prime members can get 20% Back at Amazon.com on select holiday gift items when they use their Amazon Prime Rewards Visa Signature Card. That’s an additional 15% Back on top of the everyday 5% Back they already earn at Amazon.com.
The list of eligible items can be found here (the list of eligible Pentax gear can be found here). Pretty much all Rokinon lenses are also eligible (see all brands).