We are not, we've got a full-on shopify site that we put on a subdomain, update the cart via the storefront API, and our checkout link takes you to the shopify

We are not, we've got a full-on shopify site that we put on a subdomain, update the cart via the storefront API, and our checkout link takes you to the shopify checkout. Headless, AFAIK, is a higher pricing tier. We're just sorta hacking the standard plan.