Eventually you're going to want to monetize your Alexa skill, and one way to do that is through In Skill Purchases (ISP). Mark and Allen go over the different kinds of ISPs available and how to use them. (If you're looking for how to do this on the Google Assistant - tune in next week.)  

For more about In Skill Purchases, see https://developer.amazon.com/en-US/docs/alexa/in-skill-purchase/isp-overview.html