Do I get HTTPS (an SSL certificate) on my custom domain?

No, you don't need to buy one. When you connect a domain under Settings → Custom Domain, WCart sets it up through Cloudflare, and Cloudflare issues the SSL certificate automatically.

1. Click Change domain, then set the nameservers shown under DNS setup at your registrar.
2. Once they have propagated, click Verify & activate.
3. Your store is then served over https:// on both the bare domain (mybrand.com) and its www version.

Your Current domain is shown with https:// on the same page. Until verification succeeds, your store keeps serving on its existing secure address. The Custom Domain plugin must be active to see this page.