It's amazing how much lock-in you get by developing ecommerce sites with WP for clients. Her site was recently down, just didn't load. She paid a few hundred for the agency to take a look, they said they updated plugins and "removed viruses" and all is good again. If you're someone not technical or without a huge sum of money to pay someone to replicate the functionality to migrate off WP you're on the hook for the life of business.
You can migrate hosts but that's about all the freedom you have, paying Shopify 20-30 USD / mo is nothing compared to what you'll have to eventually pay with WP if you build your business around it IMHO.