Several plugins allow adding a store functionality to a WordPress site without back end coding. The following two plugins are popular and well supported:
wooCommerce is a great WP plugin that makes it very easy to integrate a full store in a WP site, complete with product pages, shopping cart and checkout.
If you just need a shopping cart that links to PayPal WordPress Simple Paypal Shopping Cart plugin is lightweight and easy to use. It allows you to add an ‘Add to Cart’ button for your product on any posts or pages.