WooCommerce - Add extra weight to all cart items in a specific shipping class - Find out more here -> https://silicondales.com/tutorials/woocommerce/add-additional-shipping-weight-to-cart-items-in-a-shipping-class/

functions.php

// By Robin Scott of Silicon Dales
// Add 6oz extra weight to all cart items in specific shipping class
// More information here: https://silicondales.com/tutorials/woocommerce/add-additional-shipping-weight-to-cart-items-in-a-shipping-class/
add_action( 'woocommerce_before_calculate_totals', 'rscott_add_custom_weight', 10, 1);
function rscott_add_custom_weight( $cart_object ) {
    if ( (is_admin() && ! defined( 'DOING_AJAX' ) ) || $cart_object->is_empty() )
        return;
    if ( did_action( 'woocommerce_before_calculate_totals' ) >= 2 )
        return;
	$additionalweight = 6;
    foreach ( $cart_object->get_cart() as $cart_item ) {
		$shipping_class = $cart_item['data']->get_shipping_class();
		if ($shipping_class == 'shipping-class-slug' && !empty($original_weight)){
			//get the current weight
			$original_weight = $cart_item['data']->get_weight();
			// calculate new weight
			$new_weight = $original_weight + $additionalweight;
			// Set new weight
			$cart_item['data']->set_weight( $new_weight );
		}
    }
}