Google is just handing out value lately: first Nexus owners who use Android Pay get a $20 Best Buy gift card, then buying Chromecast gets you a free movie rental, and now Chromecast buyers get $20 Google Play credit. If we didn't know better, we'd think this is exploitable somehow.
The website notes the offer only works in the US, but the Google Store Twitter says it works in Canada too, so give it a try if you live in the promised land. The other key restriction is that it expires on January 2. For all the nitty gritty, hit the source.