The verdict
If you sell physical or digital products and the shop is the business, pick Shopify. Checkout, payments and orders work from the first day, and someone else keeps the servers running.
If you want to test an offer, collect a waiting list or run a campaign page, pick Carrd. It is quick to set up and cheap to be wrong with.
Many teams end up with both. A Carrd page for a campaign or a test can send people to the shop, and the shop does the real selling. That is a sensible split.
What I'd avoid is stretching either one. Carrd is a poor home for a growing catalogue. Shopify is more than you need for a single page, and its costs build up through apps, themes and payment fees, so price your real set-up before you start.