Even when the eCommerce platform itself is free, you will have to pay for a domain, web hosting, payment processing fees, and other extension costs.
With that said, your goal should be to find an eCommerce platform that helps you get started at the lowest to no cost, without putting severe limitations on how you run or grow your business.
You would also want the platform to be easy to use, full-featured, with the ability to accept payments using your preferred payment methods.
