Add shipping surcharge for one class of items in cart

add-surcharge-for-shipping-class-items.php

add_filter( 'woocommerce_package_rates', 'add_shipping_option_base_fee', 10, 2 );
function add_shipping_option_base_fee( $rates, $package ) {

    foreach( $rates as $key => $rate ) {
        // reset defaults
        $class_c_items = 0;

        // Do not apply base fee if not the right option
        if( $rate->id !== 'betrs_shipping:1-1' ) continue;

        // find items belonging to a specific shipping class
        foreach( $package['contents'] as $key => $values ) {
            $item_sclass = $values[ 'data' ]->get_shipping_class();
            if( $item_sclass == 'class-c-slug' ) {
                $class_c_items += $values['quantity'];
            }
        }

        // Add $1 surcharge for every 5 Class C items
        if( $class_c_items > 0 ) {
            $rates[ $key ]->cost += ceil($class_c_items / 5) * 1;
        }
    }
   
    return $rates;
}

It works. To use num items in cart line 15 should be: $class_c_items += $values['quantity'];

Good catch, thank you! I have updated the code accordingly.