5 Ways You're Overpaying Your Vendors (and a Checklist to Stop It)

The Problem Is More Common Than You Think

According to Stampli, up to 66 percent of invoices contain some form of error. The errors range from minor rounding discrepancies to material overcharges. The bigger problem: according to ResolvePay, AP professionals catch only 39 percent of invoice errors during their standard review process. The rest get paid.

SAP Concur estimates that 1.29 percent of invoices are duplicates, with an average duplicate invoice value of $2,034. For a business processing 450 invoices per month, that is roughly six duplicate invoices a month, or over $12,000 in potential duplicate payments. Twenty percent of businesses report experiencing overpayments in 2024.

These are not edge cases. They are the predictable result of manual review applied to high volumes of documents. Here are the five most common overpayment categories, and what each one actually looks like in practice.

1. Duplicate Invoices

A vendor sends the same invoice twice. Or the same invoice arrives by email and by mail. Or an invoice is submitted, a reminder is sent, and both get processed as separate payments. In each case, without a system tracking prior payments by vendor and invoice number, both get paid.

Duplicate payments are the most preventable category of overpayment because they are entirely mechanical to detect. If you have a record of previous payments, a duplicate shows up as a payment to the same vendor for the same amount within a short time window. The problem is that this matching step is skipped, is done inconsistently, or is handled by memory rather than process.

At 1 to 3 percent of supplier payments annually, as Precoro reports, duplicate/miscalculated spend is a meaningful leak even for businesses with modest vendor spend. A company paying $500,000 per year to vendors is potentially losing $5,000 to $15,000 to duplicates alone.

Recovery is possible if you catch duplicates before payment clears, or if you catch them quickly enough to dispute a charge. After 60 to 90 days, many vendors will not issue a credit without a fight.

2. Pricing Drift from Contracted Rates

You negotiate a contract with a vendor. Pricing is set. Then, months later, invoices start coming in slightly higher, line by line. The vendor changed their pricing structure, introduced a new fee, or simply adjusted rates upward. If no one is comparing current invoices against contract terms, the drift goes unnoticed.

This is common in industries with frequent price updates: construction material suppliers adjust pricing regularly. Property management maintenance contractors add fuel surcharges. Cleaning companies revise hourly rates with each contract renewal, sometimes without clearly communicating the change.

A 3 to 5 percent price increase across your top 10 vendors sounds small. On $200,000 in annual vendor spend, it is $6,000 to $10,000 per year leaving quietly. The fix is periodic reconciliation: pull your current invoice line item prices for major vendors and compare them against the contract or original quote. Do this once per quarter for high-spend vendors.

3. Tax Overcharges

Sales tax errors on vendor invoices fall into several categories. Wrong rate for the jurisdiction. Tax charged on exempt services. Tax applied to goods or materials that qualify for an exemption your business has not communicated to the vendor. And in some cases, plain calculation errors.

Construction companies purchasing raw materials may qualify for sales tax exemptions in their state. Property management companies often pay tax on maintenance services that are actually labor-only and exempt. Bookkeeping firms buying software subscriptions may be able to apply technology exemptions depending on the state.

The problem is that most businesses do not have a current record of their tax exemption certificates, do not always provide them to vendors, and do not check whether vendors have applied them correctly. The result is consistent overpayment on a line item that never draws attention because it looks like a routine tax charge.

Audit your exemption status with your accountant, make sure your certificates are current and on file with each relevant vendor, and review recent invoices from those vendors to confirm the exemptions are being applied.

4. Missed Early Payment Discounts

Only about 15 percent of invoices get paid within early payment discount windows, according to APQC data. The primary reasons: invoices are not processed fast enough to meet the 10-day window, no one is tracking which invoices have discount terms, or the payment approval process adds too many days of lag.

This is a category where manual processing directly costs you money. If an invoice sits in an inbox for three days before being opened, three days in a review queue, and two more days in payment processing, you have spent a week before the payment even goes out. With a 10-day window, there is no room for that kind of delay.

Start by identifying which vendors offer early payment terms. Build a simple tracking mechanism, even a spreadsheet, that flags any invoice with 2/10 net 30 terms for priority processing. Consider negotiating these terms explicitly with your top vendors if they are not already offered.

5. Quantity and Unit Price Errors

The invoice says 100 units at $5.25 each. You ordered 80 units at $4.75 each. The difference on one invoice is $125. Across a year of supply orders, these discrepancies compound. They are not always the result of bad faith on the vendor's part. Picking errors, unit-of-measure confusion (each vs. box), and data entry mistakes on the vendor side happen constantly.

The solution is a simplified three-way match: compare the invoice to what you ordered and what you actually received. Large businesses do this formally with purchase orders, receiving reports, and invoice matching software. Small businesses tend to skip it entirely because it feels like too much process.

A practical middle ground: for any invoice over $500, compare the invoice quantities and unit prices against either the original quote or purchase order, and against a delivery confirmation or receiving note. This takes five minutes per invoice and has a high catch rate for quantity errors. For invoices under $100, spot-check a random 10 percent each month.

The 10-Point Vendor Invoice Audit Checklist

Use this checklist before approving any invoice for payment. For high-volume operations, apply it to every invoice above a threshold amount (for example, $250 or $500) and spot-check a percentage of smaller invoices each month.

1. Check for duplicates. Search your payment history for the same vendor and amount within the past 45 days. Include slight date variations and format differences in the invoice number.
2. Verify vendor name and payment details. Confirm the vendor name, address, and payment information match your records exactly. Discrepancies can indicate a changed account number or, in fraud cases, a spoofed invoice.
3. Compare unit prices against contract or quote. For recurring vendors, check current invoice prices against the rate you agreed to. Even a small difference per unit adds up across large orders.
4. Verify quantities against receiving records. Confirm you received what you are being billed for. Cross-reference the invoice quantity against a purchase order, delivery note, or your own receiving record.
5. Check tax rates and exemptions. Confirm the tax rate matches your jurisdiction. Verify that any applicable exemptions have been applied. If you have a tax exemption certificate on file with this vendor, confirm it is reflected on the invoice.
6. Confirm payment terms match your agreement. Net 30, net 60, 2/10 net 30. Make sure the terms on the invoice match what you agreed to, and flag any early payment discount terms for priority processing.
7. Verify the math. Check that line item extensions (quantity x unit price) are correct, that subtotals match, and that the total invoice amount matches the sum of all charges. Errors in vendor-side spreadsheets happen.
8. Confirm GL coding. Make sure the invoice is coded to the correct expense category and cost center. Miscoding creates reporting problems downstream and can affect tax treatment.
9. Look for unexpected fees or surcharges. Fuel surcharges, handling fees, rush charges. Check whether these were disclosed in your contract or quote. If not, question them before paying.
10. Verify the invoice date is reasonable. An invoice dated months in the past may indicate a late submission that could affect your cash flow planning, or it may indicate a potential duplicate or fraudulent submission.

Making It Stick

A checklist is only useful if it is used consistently. The challenge for most small businesses and bookkeeping firms is that invoice volume is high and margins on the time spent per invoice are thin.

The practical approach is to tier your review. High-spend vendors and invoices above a dollar threshold get the full 10-point check. Recurring, low-risk invoices from trusted vendors with a consistent history get a lighter review focused on the top three items (duplicate check, amount, and any unusual fees). Spot-checking a random 10 percent of all invoices each month gives you ongoing data on where errors are concentrated.

Over time, you build a clearer picture of which vendors have cleaner invoicing practices and which ones require closer scrutiny. That knowledge shapes how you allocate review effort, and it gives you grounds for a direct conversation with vendors whose error rates are consistently high.

The 61 percent of errors that slip through AP review do so because manual review at scale is inconsistent. A systematic process, even a simple one, catches far more than an ad hoc one. Start with the checklist above, track what you catch, and adjust based on what you find.