本來 Heroku 提供的 SSL certificate 是要收費的，現在則是提供 Let's Encrypt 的免費版本，而且提供自動更新 (不過必須是收費的 Dynos)：「Announcing Free and Automated SSL Certs For All Paid Dynos」。
Heroku has always made it easy to add SSL encryption to web applications — today’s release of ACM extends that further to automatically generate a TLS certificate issued by Let’s Encrypt for your application’s custom domains.
細節的操作可以參考「Automated Certificate Management」這邊的說明。