How to Catch Supplier Overcharges Before You Pay

Why overcharges survive review

Almost every overcharge that reaches a payment run shares one property: the invoice looks fine on its own. Quantity times price equals the extension, the totals foot, the tax is right, the formatting is professional. The problem is not visible on the invoice; it is only visible against another document. The contract that set a lower rate. The quote whose volume tier the order qualified for. The PO whose freight terms said prepaid. Last month's invoice with the same amount on it.

That disagreeing document usually lives somewhere else: a contract PDF in a shared drive, a quote in a buyer's inbox, a PO in the ERP. Your ERP is a system of record; it stores the data you enter. It does not read the contract PDF, and it does not compare this week's invoice against the quote email from March. So the reviewer approves the document in front of them, which is internally consistent, and the disagreement never surfaces.

One more thing worth saying plainly: most overcharges are not fraud. They are drift. A price set correctly in January that crept by Q3. A surcharge added during a cost spike that never came off. A freight billing habit nobody re-checked against the PO terms. Drift is still your money, and the checks below catch drift and fraud the same way, because both leave the same document disagreement behind.

The five overcharge classes, and which two documents disagree

Contract price creep

The pair: the invoice against the contract, and against the vendor's own invoice history. For every line covering a contracted item, compare the invoiced unit price to the rate the contract sets for that date. Where no contract exists, compare it to the price you last paid for the same part. Creep works in small increments precisely because each individual invoice stays under the amount anyone scrutinizes; the disagreement is obvious only when the contract or the history sits next to the invoice. When the invoice disagrees with the PO itself rather than the contract, that is its own diagnostic, covered step by step in what to do when the invoice price is higher than the PO.

Misapplied volume and tier pricing

The pair: the invoice against the quote or contract that sets the tiers, joined by the PO quantity. The quote said one price through 999 units and a lower price at 1,000 and up; the PO ordered 2,500; the invoice billed the base tier anyway. A second variant applies the tier per shipment when the agreement sets it on cumulative volume, so every partial shipment bills as if it were a small order. The check reads the tier the quantity actually earned off the quote, then compares it to the invoiced unit price. This class hides well because the invoiced price is a real quoted price, just the wrong row of the table.

Duplicate and split invoicing

The pair: the invoice against every prior invoice, and the sum of invoices against the PO. Duplicates arrive four ways: the same invoice number submitted twice; the same amount under a new number; a near-duplicate with transposed digits or a suffix added; and a rebill of an invoice that was already paid. Split invoicing is the inverse pattern, one PO arriving as several invoices, each sized to pass under an approval threshold, that together exceed the PO total. The check matches invoice numbers and amounts against history, then sums every invoice on each PO against that PO's total. These two classes sit inside a larger family, cataloged in 12 invoice fraud patterns you're missing.

Surcharge padding

The pair: the invoice against the PO and the contract. A fuel, energy, handling, compliance, or tariff surcharge line is legitimate only if something you signed provides for it. The check is strict on purpose: every invoice line must trace to a PO line or a contract clause, and a line that traces to neither is a question, not a payment. The two failure modes are a surcharge that was never agreed at all, and a surcharge agreed during a cost spike that kept billing after the spike ended. Tariff surcharges deserve their own scrutiny because the underlying duty rules have been moving; the manufacturer's guide to checking your tariff bill covers that math end to end.

Freight overcharges

The pair: the invoice against the PO's freight terms. The common versions: the PO says freight prepaid and an invoice adds a freight line anyway; freight is billed above the rate that was quoted; or a handling fee is stacked on top of a freight charge that already includes handling. The check reads the freight terms off the PO, then compares them to any freight or handling line on the invoice. Freight lines get less review than product lines because they are small relative to the goods, which is exactly why the habit persists across hundreds of shipments.

The manual method: ten vendors, one quarter

You do not need software to find out whether you have this problem. You need a sample that is big enough to matter and small enough to finish:

1. Rank your vendors by last quarter's spend and take the top ten. Overcharges scale with volume, so the highest-spend vendors are where the dollars are.
2. For each vendor, pull three stacks: the contract or standing quote, last quarter's POs, and last quarter's invoices.
3. Run the five checks against every invoice line: price vs contract and history, tier vs quantity, number and amount vs prior invoices, every line traced to a PO line or clause, freight vs PO terms.
4. Log each finding with the vendor, the invoice, the line, the two disagreeing documents, and the dollar difference.
5. Total the findings by class. The class totals tell you where your controls are weakest, not just what this quarter cost.

Expect the work to be tedious rather than hard; every check is a lookup and a subtraction. What the sample gives you is proof of whether the leak exists and which classes it comes from. What it cannot do is catch the next one, because a sample is a photograph and billing drift is a motion.

One overcharge, rebuilt by hand

A machined bracket is under contract at $12.79 per unit through year end. Third-quarter invoices bill the same part at $14.32. Across the quarter the invoices cover 8,400 units, so the check is one subtraction and one multiplication: 8,400 EA x ($14.32 - $12.79) = $12,852.00 billed above the contracted rate.

Notice what made it survivable. Every individual invoice was internally consistent and each one carried a few hundred dollars of drift, comfortably below anything an approval threshold would catch. The $12,852.00 exists only as a pair: the invoice price next to the contract price, multiplied by everything shipped since the drift began. That is the shape of nearly every overcharge on this list, which is why the fix is pairing documents, not reviewing invoices harder.

Sampling versus continuous checking

The ten-vendor sample answers the question once. The drift that produced the findings resumes the following quarter, on the vendors you sampled and the forty you did not. Continuous checking means the same five comparisons run on every invoice when it arrives, before the payment run, not on a sample after the fact.

Kynthar is built around this model. You forward your procurement emails to a Kynthar address, and it reads the bodies and the attachments together, no templates and no per-vendor setup. Each document is connected to the PO, contract, shipment, vendor, and item it references, so when an invoice arrives it lands next to exactly the documents the five checks need, within minutes, with the clock starting when the email arrives instead of when someone opens it. The checks themselves are registered detectors, more than ninety of them, including price creep against history, invoiced price above the contracted rate, duplicates in each of the four variants described above, and split invoicing across a PO. Every invoice is checked against its full document chain, quote to PO to acknowledgment to receipt, the chain explained in what is 5-way invoice matching. Findings land in your team's work queue, ready to assign.

Historical documents work the same way by upload, and ERP data arrives by export: export from your ERP, and Kynthar ingests it and connects it to the documents your ERP never reads.

The arithmetic is checkable, so check it

Every check in this article reduces to two documents and a subtraction. That is worth holding onto as a standard, because it means you never have to trust a flag: you can open both documents and redo the math yourself, line by line. A finding that cannot show you its two disagreeing documents is not a finding.

Hold Kynthar to the same standard. The deterministic engine behind these checks is the same one that priced $2.32 million of duty across 532 entry lines and reconciled the total to the dollar. Reconciling to the dollar is the bar, on duty math and on overcharge math alike, and it is a bar you can verify from your own documents any time you want to.