sub price{
    my $self = shift;

    my $base_price = $self->{_quantity} * $self->{_item_price};
    my $quantity_discount = max( 0, $self->{_quantity} - 500)
                            * $self->{_item_price} * 0.05;
    my $shipping = min( $self->{_quantity}
                        * $self->{_item_price} * 0.1, 100.0);

    return $base_price - $quantity_discount + $shipping;
}