How do I add a cart?

It is not possible to add a cart on the form as this feature is not supported by jotform. However, you can still setup a payment field in your form. Simply follow this guide to set it up: Setting Up Your First Order Form. If you want to create a full-fledged online store with a cart, you may take a look at the Prestashop or Shopify websites which should fit in your requirement. 

If you need further assistance, please let us know.