Auditing your demurrage and detention invoice: line items that get billed twice

The invoice arrives with the container paperwork: $4,400 (illustrative) for eleven days of demurrage plus nine days of per diem on a shipment of bike lights. You remember the container sitting at the terminal for about four days, and the drayage driver hauling it away on day nine. Between your memory and that number sits the most recoverable money in importing — and since 2024 there are federal billing rules that put the burden of documentation on whoever issued the invoice.

This is the audit: the line items that get billed twice or run on the wrong clock, how to recompute the entire bill from the terminal's own dates, and how to file a dispute that actually gets answered. Every number below is illustrative — your rates come from published tariffs, your dates from the terminal's own container records. If you need the definitions first, the explainer on how demurrage and detention accumulate covers what each fee is and when its clock starts; this article stays on the invoice itself.

Why these invoices now come with rules attached

Demurrage and detention billing generated enough disputes that the Federal Maritime Commission issued binding billing standards, effective May 28, 2024 (Federal Maritime Commission rule on demurrage and detention billing practices, accessed November 2026). Three parts matter to you as the payer:

• You can only be billed if you're the "properly billed party" — the shipper, consignee, or whoever actually holds the contractual relationship for the freight. An invoice chasing a party who never booked the shipment fails that test.
• The invoice must carry the fee period dates — when the chargeable period started and ended, the free time allowed, and enough detail to recompute the bill. An invoice you can't audit is an invoice that doesn't meet the standard.
• Improperly billed amounts must be refunded once the issuer's own review finds them improper.

None of that shrinks the bill by itself. What it does is guarantee that the raw material for an audit is on the document — the dates you'd otherwise have to beg for are now a compliance requirement.

The six patterns that produce a wrong invoice

The first three are the classic doubles. The rest are clock-setting errors, and they cost the same way — a few invented days multiplied by an escalating daily rate.

A worked audit

The container: 980 cartons of bike lights (SKU BIKE-LT-03), Norfolk to Chicago, landed last month. The dates, pulled from the terminal's event log and the empty-return receipt (all illustrative):

• Available for pickup at the terminal: October 1
• Terminal tariff free time: 5 working days, so the last free day is October 6
• Outgate — the container left the terminal: October 9
• Returned empty to the carrier's yard: October 16

The invoice as received (illustrative rates from the carrier's escalation schedule):

Now the recompute, line by line.

Demurrage. The invoice started the clock on October 1 and ran it eleven days — treating day one as chargeable and never stopping at outgate. The correct clock: chargeable from October 7 (the day after last free day) through outgate on October 9. That's 3 chargeable days × $175 = $525 (illustrative tier-one rate).

Per diem. The invoice started detention on the same date as demurrage, which bills the same days twice — a container sitting inside the terminal cannot simultaneously be out of the terminal. The correct clock: detention starts at outgate on October 9, the carrier allows 5 free days, so the chargeable days are October 15–16. That's 2 days × $150 = $300 (illustrative).

Correct total: $825. Invoice: $4,400. Difference: $3,575 (illustrative throughout). Notice where the gap came from: two clock moves — starting the clock a week early and never stopping it — rather than any single exotic line. That's typical. An audit like this takes about an hour, and the recovered amount regularly exceeds what the container cost to freight in the first place.

The audit checklist

Run this against every demurrage or detention invoice before you pay it:

1. Get the dates from the terminal, not the invoice. The container number entered on the terminal's tracking page gives you the availability date and the outgate scan. Those two dates are the spine of the audit.
2. Pull the terminal's tariff free-time line for your commodity and destination — how many free days the terminal grants, and whether weekends or holidays extend them.
3. Recompute demurrage: from the last free day through outgate, inclusive, and never beyond outgate.
4. Recompute per diem: from outgate through empty return, minus the carrier's free days.
5. Delete overlap days. Any day billed as both demurrage and per diem is billed twice by definition.
6. Compare invoices if more than one arrived. Cross-check the carrier's bill and the forwarder's or NVOCC's bill day by day; duplicated days get claimed once.
7. Check the availability date. Nothing is billable before cargo was actually available; a clock that starts on the vessel arrival date instead is starting early.
8. Reprice from the published tariff, not from whatever rates the invoice used. Carriers and marine terminal operators publish their tariffs, and the FMC requires those charges to be publicly accessible (Federal Maritime Commission tariff requirements, accessed November 2026).
9. Attach the recompute to the dispute. A number with dates behind it gets processed; a feeling does not.

Filing the dispute that gets answered

One email, structured so the issuer can process it without follow-up questions:

Subject: Demurrage/detention dispute — B/L [number], container [number]

Per the invoice dated [date], we dispute the following line items: [list].

Recomputation from the terminal event record: available [date]; last free day [date]; outgate [date] (terminal scan attached); empty return [date]. Free time per [terminal/carrier] tariff [reference]: [X] days.

Corrected charges: demurrage [N] days × [rate] = [amount]; per diem [N] days × [rate] = [amount]. Total [amount] versus invoiced [amount]. We request a corrected invoice or refund of [difference] within [15] days. Attached: terminal tracking screenshots, tariff pages, both invoices.

Send it to whoever issued the bill, and to your forwarder if they sit in the middle. Issuers must maintain the documentation behind these invoices under the FMC's billing rule, which is exactly what makes a documented recompute hard to wave off. If a valid dispute stalls, the FMC maintains an informal complaint process for this fee category (Federal Maritime Commission, accessed November 2026) — most disputes never need it, but knowing it exists changes how seriously the first letter gets taken.

When the audit leaves a bill near the cargo's value

Sometimes the recompute stands and the number is still enormous — a long customs hold, an exam, a fee spiral. At that point the question stops being "is this bill right" and becomes "is this cargo worth keeping," which is a different decision with its own arithmetic: the walk-away calculation is covered in the abandonment decision guide. And the container costs more than fees while it sits: those SKUs sell nothing, and that quiet cost is the same arithmetic as the cost of running out of stock.

Domestic trucking has its own wait fee, with different rules and much smaller stakes — the dock-side version is covered in driver detention fees.

Common questions

How are demurrage charges calculated? Free time starts when cargo is made available for pickup and runs for the days set in the published tariff (often three to five working days at U.S. terminals); after that, a per-day charge applies on an escalating schedule. Billing must stop when the container is outgated, and the invoice must show the start and end of the chargeable period (Federal Maritime Commission rule, accessed November 2026).

What's the difference between demurrage and detention on my invoice? Demurrage covers the container sitting at the terminal past free time; detention (or per diem) covers the container out of the terminal past its return date. The two clocks must never run on the same day — overlap between them is the most common double-bill, and the full fee mechanics are in the demurrage and detention explainer.

Who is allowed to bill me demurrage? Under the FMC's billing standards, only the "properly billed party": the shipper, the consignee, or the party that actually holds the contractual relationship for the shipment. An invoice sent to a party that never contracted for the freight fails the rule, and every invoice must carry the fee-period dates and rate detail needed to verify the charges.

How long do I have to dispute a demurrage invoice? There is no single federal deadline; the claim window lives in the carrier's or terminal's tariff, and some are short — so file within days of receiving the invoice. A written dispute with a documented recompute attached is the strongest form. If it stalls, the FMC's informal complaint process exists for this fee category.

What documents prove a double bill? Three things: the terminal's container event record (availability date and outgate scan), the published tariff's free-time line, and the empty-return receipt. With those, the whole invoice recomputes on paper — any day billed outside the corrected windows, or billed as both demurrage and per diem, is recoverable.

Next step

Pull the last demurrage invoice you paid without questioning it, enter the container number on the terminal's tracking page, and recompute one line using the worked example above as the worksheet. If the numbers do not reconcile, send the dispute email this week — and the reason a stuck container hurts more than the fees is in the cost of running out.