AbanteCart is opensource and has Softaculous install scrip in c-panel. It allows you to create a script from a product or category and display it in you post or page block. Below is the HTML script generated in AbanteCart for a single product and the code inserted in post. Notice the cart icon on the right. When you click it a pop up shows a fully functional cart with checkout. You have options to choose what will be in the scrip either for HTML or embed code as a link that opens up a new page with product and cart icon.

I have it installed on the same server as my UNA site without any third party services like Shopify and uses the same Stripe Merchant Account as my membership site does. It is also a multi-cart script which means you can set up a separate store for any of your members to have their own e-commerce site with their own credit card merchant accounts.

Will it also require all users to create an abantecart account separately in order to use the service on your una site?