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。