Easy Cart was architected for small shops to limit technical debt and overhead. This simple single file cart plugin leverages the content management features of Wordpress.
Shop keeper can use add to cart buttons on existing content landing pages. Use existing post and categories for merchandising. Display the cart using shortcode on the page slug of their choice. Also further customize the page template for the slug to their desired state.
Easy Cart plugin for Wordpress can be easily extended with your custom checkout by creating a page template for slug you used to display the Easy Cart. For example if you use /cart as your page slug then you can modify pagetemplate-cart.php in your Wordpress theme folder.