Hand most people a 3PL quote and they go straight to the "cost per order" line. I go to a different line: the labor charge. It's the vaguest line on the whole quote — and the one most likely to cost you real money.
In 2023 I helped a pet-supplies client negotiate with two warehouses in Rancho Cucamonga. Storage rates and per-order fees looked nearly identical. The difference was hiding in the labor line: one billed per unit, the other per hour. The client liked the per-unit one — "simpler," he said — and was about to sign. I told him to ask one question first: what exactly counts as a "unit." Turns out their unit was per order line. His average order had 3.2 lines. The "simple" quote came out nearly forty percent more expensive than the hourly one.
So before you compare prices, figure out what that line is actually charging you for.
What that "labor charge" line is really charging you for
There are only two billing models: per unit or per hour. Neither is a scam. The traps are in the definitions.
Per unit starts with the obvious question nobody asks: what is a unit? The industry runs three versions — per order (one flat fee per order), per unit (per item shipped), and per line (per SKU line on the order). Take an order with five items across three SKU lines and the three methods produce wildly different totals. It gets worse in the fine print: does processing a return count as a unit? What about labeling, kitting, quality inspection? During peak season, when one order ships in two boxes, is that one unit or two? The quote won't mention any of this. The monthly invoice will.
Per hour has a different trap: arguing about productivity. The warehouse tells you the packing station handles a certain number of units per hour. You think that number looks slow. They say your product is fiddly to pack. Without an agreed productivity baseline and a review mechanism in the contract, you will lose that argument every time. Three more details to check: is there a daily minimum billable hours clause — you only needed two hours of work in the slow season but the invoice says four; are overtime and holiday rates billed separately; and are indirect labor like supervisors and QC included in the hourly rate or stacked on top?
I've watched people lose money on both models. The per-unit losers never nailed down the definition of a unit, then discovered returns and relabeling were billed as extra "units." The per-hour losers had no productivity baseline in the contract, so the warehouse worked at its own pace while they stared at the invoice.
Running the numbers
Which model wins depends on volume. Here are my rules of thumb by scale — treat them as directional, and negotiate your actual numbers into the contract.
A few hundred orders a month, volume still shaky: hourly usually wins. At this stage per-unit rates run high and often come with minimums — a quiet month means the minimum charge costs you more than hourly would have. But hourly demands that you watch productivity. Go stand at the packing station once and time it yourself, so you have your own baseline.
A few thousand orders a month, reasonably steady: per unit usually wins. Stable volume is negotiating leverage, the warehouse will sharpen the rate, and per unit makes costs predictable — budgeting gets easy. At this stage the whole game is locking down the definition of a unit (see below).
Tens of thousands a month: per unit with tiered pricing. At this scale, negotiate step-downs — the rate drops past certain volume thresholds — and sign a separate addendum for peak-season labor instead of letting holiday surcharges eat your margin.
Direction alone doesn't convince anyone, so let's run the math. The numbers below are purely illustrative, to show how the billing method changes the total — they are not any warehouse's real rates. Say your average order has 3.2 lines. Warehouse A quotes per line at 0.80, returns included. Warehouse B quotes per order at 1.50, with returns billed separately at 1.00 each. At 5,000 orders a month with an 8% return rate: Warehouse A costs 5,000 × 3.2 × 0.80 = 12,800 in labor. Warehouse B costs 5,000 × 1.50 plus 400 returns × 1.00 = 7,900. "Only eighty cents a line" sounds cheap until you multiply by the lines. That's why you ask what a unit is before you compare prices.
At the negotiating table, get these sentences into the contract verbatim: whether the billing unit is per order, per unit, or per line; whether returns processing, labeling, and inspection are included, and at what price if not; whether there are minimum charges or minimum billable hours; for hourly billing, what the productivity baseline is and how often it's reviewed; how long the rates are locked and when they can be renegotiated.
Don't dance around it — ask directly: "If I only ship 800 orders this month, how does the minimum charge work?" Watch how they answer. Hesitation means the minimum clause has teeth. Then ask: "How is labor billed during returns season, say January?" Plenty of warehouses quote beautifully for normal months and reclassify January returns as "special handling" at a premium. Ask now, or the January invoice will teach you the hard way.
My verdict is simple. Stable volume, simple SKUs, standardized work: per unit is the least stressful option — provided the definition of "unit" is nailed down in writing. Volatile volume with complex work like kitting and inspection: hourly is fairer — provided the productivity baseline is in the contract. And there's a third case where neither fits cleanly: wild volume swings, thousands of SKUs, frequent kitting. For those clients I usually recommend starting hourly for three months, collecting real productivity data, and then negotiating per-unit rates with the numbers as leverage. The worst outcome is signing with none of these conditions met, then getting angry at the invoice every month.
Before you renew with your 3PL or switch warehouses, answer me one question: what exactly is the "unit" you're paying for right now? If you can't answer that, you haven't really read your quote.
