PHP Shopping Cart #1: WooCommerce
WooCommerce is the most popular free e-commerce solution nowadays. Built as a WordPress plugin, it is estimated that around 25% of online stores are based on WooCommerce.
Simplicity is the main advantage of WooCommerce, making it suitable for small businesses and startups. Installation is a one-click task, a getting started wizard is provided, and initial configuration (currencies, taxes, shipping, payment methods, and so forth) is very easy. Also, it has the same admin interface as WordPress and that interface is responsive by default.
However, although the WooCommerce basic features are free, there are many paid extensions that offer additional features. For example, if you want to integrate UPS shipping, import products from CSV files, or allow customers to create wishlists, be prepared for additional cost.