Woocommerce add to the purchase value interest above the 6 installment

I would like to add a fee on the total purchase in Woocommerce after the user chooses the number of parcels on the card greater than 6. Is this correct my line of thinking?

    <?php 
            $parcelas = $_GET['installment'];
            globals $woocommerce;
            if ( $parcelas == $number > 6 ) { add_action( 
    'woocommerce_cart_calculate_fees', 'custom_fee_based_on_cart_total', 10, 1 );
   function custom_fee_based_on_cart_total( $cart_object ) {
   if ( is_admin() && ! defined( 'DOING_AJAX' ) ) return;
   // The cart total
  $cart_total = strip_tags( wc_price( $installment['installment_amount'] / 100 
  * 1.0526 ) ) ; 

  if ( $fee != 0 ) 

  $cart_object->add_fee( __( "Gratuity", "woocommerce" ), $fee, false );
                                    } ;} ?> 
            <?php endforeach; ?>
        </select>