<script>  
  function CartContainsOnlyUpsells(cart) {  
    var cartWithoutUpsells = cart.items.filter(item => item.properties?.__upcartUpsell == null);  
    return cartWithoutUpsells.length === 0;  
  }  
  
  function ToggleCheckoutButton(shouldBeEnabled) {  
    var checkoutButton = window.upcartDocumentOrShadowRoot.querySelector(".upcart-checkout-button");  
    var warning = window.upcartDocumentOrShadowRoot.querySelector(".upcartUpsellOnlyWarning");  
  
    if (checkoutButton && warning) {  
      if (shouldBeEnabled) {  
        checkoutButton.classList.remove("upcart-disable-checkout-button");  
        warning.style.display = "none";  
      } else {  
        checkoutButton.classList.add("upcart-disable-checkout-button");  
        warning.style.display = "block";  
      }  
    }  
  }  
  
  window.upcartSubscribeCartLoaded((event) => {  
    ToggleCheckoutButton(!CartContainsOnlyUpsells(event.cart));  
  });  
</script>

<div class="upcartUpsellOnlyWarning" style="display: none;">  
  Your cart only contains upsell products. <br>  
  Please add a regular product to continue to checkout.  
</div>

.upcartUpsellOnlyWarning {  
  width: 100%;  
  text-align: center;  
  margin-top: 10px;  
  color: red;  
  font-weight: bold;  
  font-size: 15px;  
}  
  
.upcart-disable-checkout-button {  
  cursor: none !important;  
  pointer-events: none !important;  
  opacity: 0.4 !important;  
}