Force NYP products to respect "sold individually" regardless of price. Requires WC 2.6.3

wc-nyp-sold-individually.php

<?php
/**
 * Plugin Name: WooCommerce Name Your Price Sold Individually
 * Plugin URI: https://gist.github.com/helgatheviking/a8802255167751a5dd746f83cdfc8716
 * Description: Stricter enforcement of "Sold Individually" for NYP items
 * Version: 1.2.0
 * WC requires at least: 2.6.3   
 * 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
 */

/**
 * Attach hooks only when NYP is active.
 */
function wc_nyp_sold_individually_init() {
	add_filter( 'woocommerce_cart_id', 'wc_nyp_force_sold_individually', 10, 5 );
}
add_action( 'wc_name_your_price_loaded', 'wc_nyp_sold_individually_init' );

/**
 * Regenerate a unique ID for the cart item being added.
 *
 * @param string cart item key.
 * @param int   $product_id - id of the product the key is being generated for.
 * @param int   $variation_id of the product the key is being generated for.
 * @param array $variation data for the cart item.
 * @param array $cart_item_data other cart item data passed which affects this items uniqueness in the cart.
 * @return string 
 */
function wc_nyp_force_sold_individually( $cart_id, $product_id, $variation_id, $variation, $cart_item_data ) {

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

	if ( $product->is_sold_individually() && WC_Name_Your_Price_Helpers::is_nyp( $product ) ){

		$id_parts = array( $product_id );

		if ( $variation_id && 0 != $variation_id ) {
			$id_parts[] = $variation_id;
		}

		if ( is_array( $variation ) && ! empty( $variation ) ) {
			$variation_key = '';
			foreach ( $variation as $key => $value ) {
				$variation_key .= trim( $key ) . trim( $value );
			}
			$id_parts[] = $variation_key;
		}

		$cart_id = md5( implode( '_', $id_parts ) );

	}

	return $cart_id;
}