Add a variant to the cart dynamically depending on what is in the cart.

delivery.liquid.html

{{ 'api.jquery.js' | shopify_asset_url | script_tag }}

<script type="text/javascript">
  var variant = 231075270, // delivery item
      count = 0, existing = 0, cart = {{ cart | json }};
  
  var returnToCart = function(line_item) { 
    window.location.href = '/cart';
  };
  
  for (var i = 0; i < cart.items.length; i++) {
    if (cart.items[i].id != variant) {
      count += count + cart.items[i].quantity;
    } else {
        existing = cart.items[i].quantity;
    }
  }
  
  // add delivery variant to match number of items in the cart
  if (count != existing) {
      if (existing > 0) {
          Shopify.changeItem(variant, count, returnToCart);
      } else {
          Shopify.addItem(variant, count, returnToCart);
      }
  }
</script>