The problem might be that subdomains are often assigned to people/groups affiliated with the university who can then run their own mail servers under that subdo

The problem might be that subdomains are often assigned to people/groups affiliated with the university who can then run their own mail servers under that subdomain. This makes it trivial to give an email address ending in .edu to people who aren't students. Since in my experience Amazon's customer support people are actually people, they might be able to make an exception for your particular domain if you contact them.