"Hi, I noticed your website does not support HTTPS. Have you heard about Let's Encrypt? Let’s Encrypt is a free, automated, and open Certificate Authority. [...]"
Ideally available in multiple languages as well.
LE imposes a requirement to renew certificates every 90 days, which is a nuisance. Logging-in, finding my process crib-sheet, running the update requests, updating DNS records, running the validation requests, shifting certs to servers, restarting services... all because the LE project wants to evangelise short certificate lifespans, which seems to have been bolted onto the original goal of 'encrypt all the things' because of someone's personal conviction.
I do wish they'd offer a longer optional lifespan. Even 365 days would be a huge improvement. Until then I really can't recommend it to less-technically-inclined colleagues; I just tell them go and buy a one-year cert from Gandi, chuck it into the ssl directory and forget about it until the renewal reminder arrives 11 months later.
You're doing it wrong. There are many ways to use the ACME protocol (and Lets Encrypt), I think pretty much anyone will agree this is simply the wrong way to do it.
The ACME protocol is an API (as opposed to the CA's "upload a manually created, signed CSR, then wait for an HTML email and click the link in it and then download this file" process) so that it can be automated, and hands-off.