i'm pretty sure caddy doesn't work with cpanel.
"Caddy" looks like something you'd install on a VPS. I don't think you need it with shared hosting.
if you want to save that $7 per domain and don't want to manage your own hosting, you can always route your domains through cloudflare and use their free SSL.
I can only add how much I dislike Cloudflare: I've spent many hours of my life looking at their "verifying" page that no doubt collects as much data on me as they can for a few seconds each time. Or, as theymos puts it:
The Internet is seriously flawed if everyone needs to huddle behind these huge centralized anti-DDoS companies in order to survive...