Add checkout fee for shipping/payment method combination

// Add fee for specific shipping/payment method combo

add_action( 'woocommerce_cart_calculate_fees', 'mx_add_fee_to_checkout' );

function mx_add_fee_to_checkout() {
    $chosen_gateway = WC()->session->get( 'chosen_payment_method' );
    $chosen_methods = WC()->session->get( 'chosen_shipping_methods');
    $chosen_shipping = $chosen_methods[0];
       if ( ($chosen_gateway == 'cod') &&  0 === strpos($chosen_shipping, 'flat_rate:2')) {
          WC()->cart->add_fee( 'Extra Fee for COD & Flat rate', 5 );
      }
}