I'm honestly not entirely sure what the terms are, but Amazon is advertising a $10 Amazon.com credit with a subscription to HBO Now for one day only as part of their Digital Day sale (that started at 12am Eastern time, as advertised).
The $10 credit promo is advertised on their Digital Day page and the app page, but there are no details or terms listed at all. I just downloaded the app and signed up for the 1 month free trial to see if it would work and haven't received anything yet, but who knows how long it might take to get the credit or if there are any unannounced terms (like if it has to be a paid subscription instead of a trial).
So, basically, if anyone else wants to give it a try and see how it goes...
Edit: It's for paid subscriptions only, credit issued after first billing period after your free trial. Here are the terms (Thanks CrepeNuts): https://www.amazon.c...ode=17452622011