The format for the URL is https://yoursite/cart/add/productID.
The Product ID is visible on the admin side as indicated in the image below. To use this feature add “/cart/add/productid” to the end of your site's URL. The Product ID for the example to the right is 95081. The direct link to the checkout page for this example is https://demo.elevate.commpartners.com/cart/add/95081