Permite modificar la cantidad de los productos añadidos al carrito de WooCommerce desde la propia página de «Finalizar compra».

// Oculta la cadena de las cantidades junto al nombre del producto

add_filter( 'woocommerce_checkout_cart_item_quantity', '__return_empty_string' );

// Agrega el selector de cantidades
add_filter( 'woocommerce_cart_item_subtotal', 'wc_payment_amount_selector', 9999, 3 );
function wc_payment_amount_selector( $product_quantity, $cart_item, $cart_item_key ) {
    if ( is_checkout() ) {
        $product = apply_filters( 'woocommerce_cart_item_product', $cart_item['data'], $cart_item, $cart_item_key );
        $product_id = apply_filters( 'woocommerce_cart_item_product_id', $cart_item['product_id'], $cart_item, $cart_item_key );
        $product_quantity = woocommerce_quantity_input( array(
            'input_name' => 'shipping_method_qty_' . $product_id,
            'input_value' => $cart_item['quantity'],
            'max_value' => $product->get_max_purchase_quantity(),
            'min_value' => '0',), $product, false );
            
            $product_quantity .= '<input type="hidden" name="product_key_' . $product_id . '" value="' . $cart_item_key . '">';
        }
        
    return $product_quantity;
}

// Detecta el cambio de cantidad para recalcular los totales
add_action( 'woocommerce_checkout_update_order_review', 'wc_recalculating_totals' );
function wc_recalculating_totals( $post_data ) {
    parse_str( $post_data, $post_data_array );
    $updated_qty = false;
    foreach ( $post_data_array as $key => $value ) {
        if ( substr( $key, 0, 20 ) === 'shipping_method_qty_' ) {
            $id = substr( $key, 20 );
            WC()->cart->set_quantity( $post_data_array['product_key_' . $id], $post_data_array[$key], false );
            $updated_qty = true;
        }
    }
    if ( $updated_qty ) WC()->cart->calculate_totals();
}