Adds the feature to add multiple products on WooCommerce Cart

add-more-to-cart.php

class add_more_to_cart {

    private $prevent_redirect = false; //used to prevent WC from redirecting if we have more to process

    function __construct() {
        if ( ! isset( $_REQUEST[ 'add-more-to-cart' ] ) ) return; //don't load if we don't have to
        $this->prevent_redirect = 'no'; //prevent WC from redirecting so we can process additional items
        add_action( 'wp_loaded', [ $this, 'add_more_to_cart' ], 21 ); //fire after WC does, so we just process extra ones
        add_action( 'pre_option_woocommerce_cart_redirect_after_add', [ $this, 'intercept_option' ], 9000 ); //intercept the WC option to force no redirect
    }

    function intercept_option() {
        return $this->prevent_redirect;
    }

    function add_more_to_cart() {
        $product_ids = explode( ',', $_REQUEST['add-more-to-cart'] );
        $count       = count( $product_ids );
        $number      = 0;

        foreach ( $product_ids as $product_id ) {
            if ( ++$number === $count ) $this->prevent_redirect = false; //this is the last one, so let WC redirect if it wants to.
            $_REQUEST['add-to-cart'] = $product_id; //set the next product id
            WC_Form_Handler::add_to_cart_action(); //let WC run its own code
        }
    }
}
new add_more_to_cart;