This MU-Plugin helps sort Predefined Products to behave a certain way when they're added to the cart. Requires WooCommerce to work. Use at your risk, enjoy! I mean, what's life without taking risks? 🚀

pekky-wc-stand-alone-cart-sorting.php

<?php
/**
 * WooCommerce Cart Stand-alone Custom Sorting.
 *
 * @package pekky-wc-stand-alone-force-sells-sorting
 * @author Precious Omonzejele (CodeXplorer 🤾🏽‍♂️🥞🦜🤡)
 *
 * @wordpress-plugin
 * Plugin Name: WooCommerce Cart Stand-alone Custom Sorting
 * Plugin URI: https://gist.github.com/Preciousomonze/f41ebbd43b751d00e86fa4207e88413f
 * Description: This MU-Plugin helps sort Predefined Products to behave a certain way when they're added to the cart. Requires WooCommerce to work.
 * Author: Precious Omonzejele (CodeXplorer 🤾🏽‍♂️🥞🦜🤡)
 * Author URI: https://codexplorer.ninja
 * Version: 1.0.0
 * Requires at least: 5.0
 * Tested up to: 5.4
 * WC requires at least: 4.0
 * WC tested up to: 4.5
 */
class Pekky_WC_Stand_Alone_Cart_Sorting {

	// The grouped Products.

	/**
	 *  Stand Alone Product id(s)
	 *
	 * Sample values: array( 23,45,43 );
	 *
	 * @var array
	 */
	protected static $wc_stand_alone_products = array();

	/**
	 * Boostrap and shoot.
	 */
	public static function init() {
		// Manipulate when added to cart.
		add_filter( 'woocommerce_add_to_cart_validation', array( __CLASS__, 'sort_product_add_to_cart_validation' ), 20, 3 );
	}

	/**
	 * Cart Item adding validation
	 *
	 * Sorts a product being added to cart,
	 * checking if its standalone
	 *
	 * @param bool $passed return true or not.
	 * @param int  $product_id Product being added to cart.
	 * @param int  $quantity quantity of product.
	 * @return bool
	 */
	public static function sort_product_add_to_cart_validation( $passed, $product_id, $quantity ) {
		$stand_alone_products = self::$wc_stand_alone_products;

		if ( in_array( $product_id, $stand_alone_products, true ) ) {
			// The standalone product was found.
			self::pekky_sort_stand_alone_products( $product_id, $stand_alone_products, true );
		} else {
			// Another product, check if a standalone product is already in the cart.
			foreach ( $stand_alone_products as $p_id ) {
				// Is this standalone product in the cart?
				if ( in_array( $p_id, array_column( WC()->cart->get_cart(), 'product_id' ), true ) ) {
					// Prevent other products from being added and reserve runtime abeg :).
					wc_add_notice(
						__( 'This Product cannot be addede because the current product(s) in the cart are standalone.', 'woocommerce' ),
						'error'
					);
					$passed = false;
					break;
				}
			}
		}
		return $passed;
	}

	// Inner functions to help run some parole.

	/**
	 * Sorts the stand alone products.
	 *
	 * @param  int   $product_id Standalone Product Id.
	 * @param  array $stand_alone_products List of standalone products.
	 * @param  bool  $add_to_cart (optional) true, add to cart, if false, removes, default is true.
	 * @return bool  false if empty field, true otherwise.
	 */
	private static function pekky_sort_stand_alone_products( $product_id, $stand_alone_products, $add_to_cart = true ) {
		if ( empty( $product_id ) || ( empty( $stand_alone_products ) || ! is_array( $stand_alone_products ) ) ) {
			return false;
		}
		$wc_cart   = WC()->cart;
		$cart_func = 'remove_cart_item';
		if ( $add_to_cart ) {
			$cart_func = 'add_to_cart';
			// Empty cart cause a standalone program is being added.
			$wc_cart->empty_cart();
		}
		$p_id = ( $add_to_cart ? $product_id : $wc_cart->generate_cart_id( $product_id ) );
		// var_dump("<br><br>".$cart_func);
		// var_dump("<br>product_id:".$product_id."<br>");
		// var_dump( "<br>function executed: ");
		if ( ! $add_to_cart ) {
			$wc_cart->{"{$cart_func}"}( $p_id );
		}

		return true;
	}

}
// Launch!
Pekky_WC_Stand_Alone_Cart_Sorting::init();