Add extra cost to flat rate shipping each 2 items from specific category in WooCommerce

Based on Add an additional cost to flat rate shipping each 3 items in Woocommerce answer code, I have made some changes to add an additional cost to flat are shipping method each 2 items (instead of each 3 items) and only when there are exclusively items from a specific category (here “T-Shirts” category).

Here’s my code attempt:

// add X amount to shipping for every 2 items added to the cart (flat rate only)
add_filter('woocommerce_package_rates', 'shipping_additional_cost_each_three_items', 100, 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 = 8.40;
    $items_count = WC()->cart->get_cart_contents_count();

    // Define/replace here your correct category slug (!)
    $product_category = 't-shirts';
    $prod_cat = false;

    // Going through each item in cart to see if there is anyone of your category        
    foreach ( WC()->cart->get_cart() as $cart_item ) {
        $product_id = $cart_item['product_id'];
        if ( has_term( $product_category, 'product_cat', $product_id ) ){
            $prod_cat = true;
        }
    }

    // Loop through the shipping taxes array
    foreach ( $rates as $rate_key => $rate ){
        $has_taxes = false;

        // Targetting "flat rate"
        if( 'flat_rate' === $rate->method_id ){

            // Get the initial cost
            $initial_cost = $new_cost = $rates[$rate_key]->cost;

            // Adding the additional cost if product in T-Shirt category after every 2 items (3, 6, 9 …)
            if ( $prod_cat ) {

                for($i = 0; $i <= $items_count; $i+=3){
                    $new_cost += $additional_cost;
                }
            }

            // Set the new cost
            $rates[$rate_key]->cost = $new_cost;

            // Taxes rate cost (if enabled)
            $taxes = [];

            // Loop through the shipping taxes array (as they can be many)
            foreach ($rates[$rate_key]->taxes as $key => $tax){
                if( $rates[$rate_key]->taxes[$key] > 0 ){
                    // Get the initial tax cost
                    $initial_tax_cost = $new_tax_cost = $rates[$rate_key]->taxes[$key];

                    // 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
                }
            }
        } 
        if( $has_taxes )
            $rates[$rate_key]->taxes = $taxes;

        return $rates;
    }
}

For “Flat rate” chosen shipping method, the code works just fine when items from “t-shirts” category are in the cart. But if there is an item that doesn’t belongs to “t-shirt” category, the rate just disappears and shows the title with no amount.

Can someone tell me where I have to place the condition ‘if category is X’, to make that code functional?

Answer

There are some mistakes in your code (like return $rates; that should need to be just before last closing bracket). Try the following revisited code instead:

add_filter('woocommerce_package_rates', 'shipping_additional_cost_each_three_items', 100, 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 = 8.40;
    $each_items      = 2; // Number of items (for additional cost)

    // Her set your category(ies) (can be term Ids slugs or names)
    $product_categories = array('t-shirts');
    $items_cat_count    = 0; // Initializing

    // Loop through cart items for the current shipping package        
    foreach( $package['contents'] as $cart_item ) {
        if ( ! has_term( $product_categories, 'product_cat', $cart_item['product_id'] ) ){
            $items_cat_count += $cart_item['quantity']; // Count items from defined category
        }
    }
    
    if ( $items_cat_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; // Get the initial cost
                $has_taxes    = false; // Initializing
                $taxes        = array(); // Initializing
                
                // Adding to cost the additional cost each 2 items (2, 4, 6 …)
                for($i = 0; $i <= $items_cat_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 in the array
                        $taxes[$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;
}

Code goes in functions.php file of the active child theme (or active theme). It should works.

Don’t forget to empty your cart to refresh shipping cached data.