PayPal sent out emails today to some customers with a code to save $50 on Nest Thermostats. Seems like you don’t need to use PayPal for the purchase, just the code that came with the email. It expires 2/1/2019. That means that you should be able to stack this with a $40 discount or 4,000 Membership Rewards points from this Amex Offer.
Also check out rebates from your local utility company. Sometimes they offer them as an incentive to be more energy efficient.