I bought Tex: The Lonesome Rider (a western graphic novel illustrated by the great Joe Kubert. $20 on ComiXology, $5 on Amazon!) and Amazon sent me an email allowing me to chose a free Marvel graphic novel. Here's the link listing the free choices. Not sure if it will work:
https://www.amazon.com/comicspromo
The thing is, Tex isn't published by Marvel. I'm not sure what the criteria was that earned me the free book. I just purchased StarSlayer from Amazon. I doubt I'll get a second free book, but fingers crossed.