A good approach could be to put all the static stuff (the sales pages, pricing info, everything that doesn't require backend) on 1 domain, and put that on S3 an

A good approach could be to put all the static stuff (the sales pages, pricing info, everything that doesn't require backend) on 1 domain, and put that on S3 and Cloudfront. That definitely won't go down, and it will be fast and take the brunt of the load. Then put the interaction stuff (the forms etc.) on a subdomain, and put that on heroku or whatever with plenty of servers.