Brooke,
I emailed Stripe about it, and they told me to try a test. Then I told them the results of that test, and this is part of what they said:
Stripe recently began to require valid certificates for HTTPS webhook endpoints and most often these issues occur when your site is missing an intermediate SSL certificate. Specifically, on the SSL Labs results page you linked, you will see some of the items in the Certificate Path section marked as “Extra download.”
The only way around that was to see if I could get the intermediate certificates included in the CA bundle from Comodo, so that the “Extra download” errors on the SSL Labs results would go away.
That’s when I got in touch with my host, who thought this was all very odd and opted to try installing Let’s Encrypt instead.
I don’t know whether PayPal would take the same view as Stripe. But I can say that, if you have the choice of whether to use PayPal or Stripe, there is absolutely no way I could recommend PayPal. Even if it’s slightly more convenient for this, you’ll soon find multiple reasons to regret ever using PayPal when Stripe is so easy.
My advice would be this. Now you’ve got the Comodo SSL certificate, try it out. If it doesn’t work (which you’ll know because you’ll start receiving email messages from Stripe) ask for a refund and install Let’s Encrypt.
And if your host makes it difficult to do this sort of thing, do yourself a massive favor and move to Lightning Base, where you’ll not only get a very fast site but the best customer service ever.