Amazon is having a promotion where if you trade-in "any qualifying game, console or accessory" by 12/16, you get a bonus $50 savings on a 1TB PS4 Pro that's shipped and sold by Amazon. In addition to this, on 12/10 the base price for the Pro is supposed to go down to $349 (according to Sony).
I have successfully completed a simple, qualifying trade-in and I'm being offered the $50 savings at checkout on a $399 Pro so I know the offer has applied to my account. If you've traded in with Amazon before, you may qualify for "instant payment" where they apply the gift card balance to your account prior to them actually receiving the game. I can also confirm that the $50 savings will also apply instantly if you qualify for that option.
Please note that Amazon is stating that the promotional savings will expire if not used by 1/31/18!
Going to drop some supporting links and pics below.