Custom “Add to Cart” URLs - not working as expected with cart & checkout blocks

[garymurray]

Describe the bug
As reported on the release post of the cart and checkout blocks - if someone is using custom add-to-cart urls on something like a landing page, and limiting the item to be sold invidually - if the customer happens to go back to the landing page and tries to add a second item (of the same thing) to their cart they get a broken error message in the cart. The shortcode used to just take them directly to the checkout page and show the error message. See screenshots below.

To Reproduce

• Create a new blank page
• Add a button, and give that button a custom add to cart url for a product
• Ensure the product you are using in the button is set to only Enable this to only allow one of this item to be bought in a single order
• Add the item to your cart - you should be redirected to the checkout page
• Then navigate back to the initial landing page with the custom button
• Add another one of the same product to your cart
• See the error message in the cart

Expected behavior

• The experience should at least be similiar or an improved version of how the shortcodes handled this - i.e. redirected back to the checkout page with a message to say you can only buy 1 item of the product.

Screenshots

Error message as shown in the cart block after trying to enter a second product:

The current message as shown in the checkout (using the shortcodes):