Pick and pack

What it means

Usually priced as a base per order plus a rate per additional unit — $2.50 for the first item, $0.35 for each one after. Some contracts price per line rather than per unit, which is a different number for the same shipment.

What goes wrong

Per-unit billed as per-line, or the base charged on every unit instead of once per order. On a high-volume month the gap compounds quietly: a $0.35 error on 40,000 units is $14,000 that never looks like anything.

The arithmetic

1,000 orders · 3,200 units
Contract · $2.50 per order, $0.35 per additional unit.

As written
  1,000 × $2.50           = $2,500.00
  2,200 × $0.35           =   $770.00
                              ----------
                              $3,270.00

Additional rate applied to every unit
  1,000 × $2.50           = $2,500.00
  3,200 × $0.35           = $1,120.00
                              ----------
                              $3,620.00

Difference                      $350.00

One month. The rate is right, the count is wrong,
and nothing on the invoice looks unusual.

How to check it

Take the order count and the unit count separately. Base × orders, plus additional rate × (units − orders). If the invoice used units for both, it is wrong.