Snippet to add fee to WooCommerce cart

add-cart-fee.php

<?php

add_action( 'woocommerce_cart_calculate_fees','woocommerce_custom_surcharge' );
function woocommerce_custom_surcharge( $cart ) {
  if ( is_admin() && ! defined( 'DOING_AJAX' ) )
    return;
	
	// ADD FLAT RATE FEE
	$surcharge = 5; // Replace 5 with your fee
	
	// ADD FEE AS PERCENTAGE OF CART TOTAL
  $percentage = 0.15; // Replace 0.15 with your percentage
  $surcharge = ceil( $cart->cart_contents_total * $percentage ); 
	
  $cart->add_fee( 'Fee Label', $surcharge, true, '' );
}

?>