I appreciate the time you've taken to write this up, and I'm sorry to see more reports of payment processors that don't use SNI. It's very unfortunate and I hope they will keep being encouraged by their customers to adopt SNI.
I just wanted to point out that the Let's Encrypt certificate itself isn't an "SNI certificate", as nothing about the certificate either requires or forbids it to be used with SNI. Whether it is used with SNI depends entirely on the web server configuration, as the rest of your message makes clear.