Let's Encrypt is nice for your web server, but using that certificate at the ELB may not be so trivial. I hope that Amazon will offer free SSL certificates for ELB. Their custom SSL certificates for CloudFront currently cost the outrageous $600/month (well, $20/day), and recently they added free SNI certificates, so, for CloudFront, they still may charge us, but, hopefully, a lot less than the crazy $600!