There are a couple of modules out there to ajaxify the add to cart form of Drupal Commerce, but I'll introduce to one that on top offers a real decoupled solution. Additionally I'll show how you can achieve a simpler version of that with only a few lines of custom JS code.
In a current project, I had to generate PDF documents based on custom entities on a Drupal 8 page. Although this has never been so easy than with Dompdf, I've stumbled across two problems which cost my some time to find out and fix.