From aboutamazon.com:
Starting December 7, any time a customer says “Alexa, thank my driver,” the driver who delivered their most recent package will be notified of the customer’s appreciation. And, in celebration of this new feature, with each thank-you received from customers, drivers will also receive an additional $5, at no cost to the customer. We’ll be doing this for the first 1 million thank-you’s received. And, the five drivers who receive the most customer “thank-you’s” during the promotional period, will also be rewarded with $10,000 and an additional $10,000 to their charity of choice.