更新（2021年）

以下代码将为每3项（3、6、9…）的固定费率装运方法增加额外成本。

您将需要更改您的运输成本与一个简单的初始成本，而不是您的公式。

您可能必须在“装运选项”选项卡下的常规装运设置中“启用调试模式”，以临时禁用装运缓存。

代码（您将在其中设置附加运费）：

add_filter('woocommerce_package_rates', 'shipping_additional_cost_each_three_items', 10, 2);
function shipping_additional_cost_each_three_items( $rates, $package ){
    if ( is_admin() && ! defined( 'DOING_AJAX' ) )
        return $rates;

    // HERE set your additional shipping cost
    $additional_cost = 1.25;
    $items_count     = 0; // Initializing
    $each_items      = 3; // Number of items (for additional cost)

    // Loop through cart items for the current shipping package        
    foreach( $package['contents'] as $cart_item ) {
        $items_count = += $cart_item['quantity']; // Count cart items for current shipping package
    }

    if ( $items_count >= $each_items ) {
        // Loop through the shipping taxes array
        foreach ( $rates as $rate_key => $rate ){
            // Targetting "flat rate"
            if( 'flat_rate' === $rate->method_id ){
                $initial_cost = $new_cost = $rate->cost;
                $has_taxes    = false; // Initializing
                $taxes        = array(); // Initializing
                
                // Adding to cost the additional cost each 3 items (3, 6, 9 …)
                for($i = 0; $i <= $items_count; $i+ = $each_items){
                    $new_cost += $additional_cost;
                }
                $rates[$rate_key]->cost = $new_cost; // Set the new cost
    
                // Taxes rate cost (if any) - Loop through taxes array (as they can be many)
                foreach ($rate->taxes as $key => $tax){
                    if( $tax > 0 ){
                        // Get the initial tax cost
                        $initial_tax_cost = $new_tax_cost = $tax;
                        // Get the tax rate conversion
                        $tax_rate    = $initial_tax_cost / $initial_cost;
                        // Set the new tax cost
                        $taxes[$key] = $new_cost * $tax_rate;
                        $has_taxes   = true; // Enabling tax changes
                    }
                }
                // set array of shipping tax cost
                if( $has_taxes ) {
                    $rates[$rate_key]->taxes = $taxes; 
                }
            }
        }
    }
    return $rates;
}

代码function.php活动子主题（或活动主题）的文件中。测试和工作。

别忘了禁用发货设置中的“启用调试模式”选项。

根据你的第二条评论回答：

您将替换此块：

// Adding to cost the additional cost each 3 items (3, 6, 9 …)
for($i = 0; $i <= $items_count; $i += $each_items){
    $new_cost += $additional_cost;
}

通过以下方式：

// Adding to cost an additional fixed cost for the 2nd item
if($items_count >= 2){
    $new_cost += 6.21; 
}

// Adding to cost the additional cost each 3 items (3, 6, 9 …)
for($i = 0; $i <= $items_count; $i += $each_items){
    $new_cost += $additional_cost;
}

<?php
$number = 13;
$all_number = 1;
for($i=1;$i<=13;$i++){
    if ($i % 3 == 0){
        $all_number = $all_number + 1;
    }
}
$price_new_data = $old_price*$all_number;
$price = 13.50+($price_new_data*$product_quenty);
?>