Hi @tdreher ,
Apparently all Shopify plans come with a free SSL certificate even 3rd party domains. Which makes sense since there's no where to actually upload a certificate within your Shopify account. Have you already added your domain to Shopify? If not you may want to do so and wait 24-48hours for your SSL certificate to be completed.
It's mentioned here in these posts by their support staff: https://community.shopify.com/c/Shopify-Discussion/Installing-a-different-SSL-Certificate/td-p/37329...
Sam - Owner @ Achieve