We originally planned to support Amazon Drive. In order to do this, we’d need access to the Amazon Drive API.
We already asked Amazon for access to their API but they denied our request. Thus, Amazon Drive – although we would like to – can’t be supported at this time.
Additional details are documented here. Should Amazon publish the API or grant us access in the future, then we would be glad to integrate Amazon Drive.
If Amazon Drive is the only cloud storage service you’d like to use with Cryptomator for iOS, you can get a refund from Apple: https://reportaproblem.apple.com