Force NYP products to respect "sold individually" regardless of price

wc-nyp-sold-individually.php

<?php
/**
 * Plugin Name: WooCommerce Name Your Price Sold Individually
 * Plugin URI: https://gist.github.com/helgatheviking/25e57fe033d719059dcf994ffe33d718
 * Description: Double check enforcement of "Sold Individually" for NYP items
 * Version: 1.0.0
 * Author: Kathy Darling
 * Author URI: http://kathyisawesome.com/
 *
 * Copyright: © 2016 Kathy Darling
 * License: GNU General Public License v3.0
 * License URI: http://www.gnu.org/licenses/gpl-3.0.html
 */
 
 add_filter( 'woocommerce_add_to_cart_validation', 'wc_nyp_sold_individually_double_check', 5, 20 );
function wc_nyp_sold_individually_double_check( $passed, $product_id, $quantity, $variation_id = '', $variations= '' ) {

    // if it has already failed no need to keep checking
    if( $passed === false ){
        return $passed;
    }

    // Get the product
    $product_data = wc_get_product( $variation_id ? $variation_id : $product_id );

    // double check against the cart contents
    if ( $product_data->is_sold_individually() ) { 
        foreach ( WC()->cart->get_cart() as $cart_item ){ 
            if ( $product_id == $cart_item['product_id'] || ( $variation_id && $variation_id == $cart_item['variation_id'] ) ) {
                $passed = false; 
                wc_add_notice( sprintf( '<a href="%s" class="button wc-forward">%s</a> %s', WC()->cart->get_cart_url(), __( 'View Cart', 'woocommerce' ), sprintf( __( 'You cannot add another &quot;%s&quot; to your cart.', 'woocommerce' ), $product_data->get_title() ) ), 'error' );
            } 
        }
    }

    return $passed;

}