Cart or Checkout Page is Removing Products from the Order

If Shopify asks you to remove an item from the cart or checkout page, it's possible that the item has no stock or is set to "not visible" on the product edit page in the Shopify admin (Shopify will not let one check out with a product set to not visible).

Visit Managing inventory (Shopify site) and Visibility (Shopify site) to view more information about these issues.