sub shop_for_rates {
    my $self = shift;
    my ($from, $to, $packages, $args) = @_;

    unless ( $from && $to && $packages ) {
        croak "shop_for_rates(): usage error";
    }
    unless ( ref $from ) {
        $from = Net::UPS::Address->new(postal_code=>$from);
    }
    unless ( ref $to ) {
        $to =  Net::UPS::Address->new(postal_code=>$to);
    }
    unless ( ref $packages eq 'ARRAY' ) {
        $packages = [$packages];
    }
    $args           ||= {};
    $args->{mode}     = "shop";
    $args->{service}||= "GROUND";
    return [sort{$a->total_charges <=>$b->total_charges} @{$self->request_rate($from, $to, $packages, $args)}];
}