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sub checkout_total {
   +shift->items->get_column("cost")->sum;
}

# Inside My::Schema::Cart or an auxillary class only
# loaded into the space on demand.
...
   my $sum = $cost_col->sum;
   return $sum;
}

# This is not exactly how it would be set up for real.
# Load order and @INC stuff requires some tweaks to
...
    item:4 -->  $ 6.72
    item:5 -->  $15.50
    item:6 -->  $63.42