There are plenty of choices which you can find in the internet. If you are using PHP, WooCommerce, Magento and PrestaShop are open source that you are free to use. Magento also offer 2 license which one is free open source and another one is a enterprise license which cost around HKD$120,000 a year.
For the past 2 years, WordPress WooCommerce has picked up some traction and now being the number 1 market share for eCommerce software and it has only one license type which is free open source. WordPress CMS has a market share of over 27% website in the globe so WooCommerce will be an obvious option for existing WordPress users who want to become eShopping site. WooCommerce is very easy to get started but it could get more complex when you want to build more than just a simple eCommerce site.
Source from google image