Paywall for submitting

USP focuses on forms and submitting post content, and leaves other types of functionality (e.g., e-commerce/shopping cart) to other, dedicated plugins. So you can use USP for the post-submit form, and whichever e-commerce plugin you would like. USP is compatible with just about any of them. Here is a post with more details to give you some ideas. Note the post is written for pro users but contains useful information for free version (and other plugins) as well.