function add_product_validation($valid, $product_id, $quantity)
{
global $woocommerce;
if (WC_Sponsorship_Product::is_sponsorship_contribution_level($product_id)) {
$woocommerce->cart->empty_cart();
} elseif (WC_Sponsorship::cart_contains_sponsorship_contribution()) {
WC_Sponsorship::remove_sponsorship_from_cart();
$woocommerce->add_error('A sponsorship contribution has been removed from your cart. Due to payment gateway restrictions, products can not be purchased at the same time.');
$woocommerce->set_messages();
// Redirect to cart page to remove subscription & notify shopper
add_filter('add_to_cart_fragments', array(&$this, 'redirect_ajax_add_to_cart'));
}
return true;
}
