使用过滤器或挂钩修改购物车总量

好吧，我正在尝试编写一个过滤器或挂钩，它将根据购物车中的商品总数来修改购物车的总量。例如，如果购物车中有三件商品，我想从总订单中扣除二十美元。下面是我到目前为止的代码，非常感谢任何帮助！

add_filter('woocommerce_cart_contents_total', 'bundle_deals');

function bundle_deals( $cart_contents_total, $cart_contents_count) {
   global $woocommerce;

   if ($woocommerce->cart->get_cart()->cart_contents_count <= 3) {
     $cart_contents_total = $woocommerce->cart->get_cart()->cart_contents_total - 20.00;
   }

   return $cart_contents_total;
}

function woocommerce_cart_subtotal(  $cart_subtotal, $compound, $obj  ){

$t = 0;
foreach ( $obj->cart_contents as $key => $product ) : 

    $product_price = $product['line_total'];    

    foreach ( WC()->cart->get_coupons( 'order' ) as $code => $coupon ) : 

            $product_price  = 19;//wpq_9522_discount( $product['line_total'], $coupon->discount_type, $coupon->amount ); 

    endforeach; 

    $t += $product_price;

endforeach; 

return $cart_subtotal;//( $t > 0 ) ? sprintf( '<s>%s</s> %s',      
$cart_subtotal, wc_price( $t ) ) : $cart_subtotal ;
}

add_filter( 'woocommerce_cart_subtotal', 'woocommerce_cart_subtotal',      99, 3 );