On AWS you can get free SSL certificates from AWS. https://aws.amazon.com/certificate-manager/
/me looks at his February bill and remembers the lesson all too well
From what I can tell, the only reasons to use alex/letsencrypt-aws (IAM certificates) instead of AWS Certificate Manager are if you're either 1, provisioning ELBs in a region other than us-east-1 and unable to wait for Amazon to roll out ACM to other regions; or 2, subsequently applying the same certificate to other resources in addition to ELB+CloudFront (e.g., a standalone EC2 instance running nginx or apache).
Unfortunately it looks as though letsencrypt-aws won't either.