Do not compare invoice totals
Two invoice totals can differ even when every unit price is unchanged. Quantity, product mix, freight, credits, and tax all move the total. Compare normalized unit cost for matched line items, then calculate the impact at the quantity you expect to buy.
Free two-invoice comparison workbook
Paste an earlier and later invoice, use stable Product IDs, and review the formula-led Comparison sheet.
Printable audit checklist
Use this final check before escalating a result to a supplier.
A six-step invoice comparison method
1. Capture the comparable fields
Record invoice date and number, supplier, supplier SKU, description, pack, comparison unit, quantity, subtotal, freight or fees, discount, tax, and currency. Keep the source files; the CRA explains the records that support tax claims.
2. Match the same commercial item
Use a stable Product ID. Do not rely only on description text. A renamed SKU may be the same product, while an identical description can hide a new pack size.
3. Normalize the unit
Convert both lines to each, kg, g, L, mL, or another shared unit. Document the conversion.
4. Choose one cost basis
Decide whether freight and fees belong in comparable cost and apply the decision to both invoices. Exclude recoverable tax if that matches your accounting policy; ask your accountant when uncertain.
5. Calculate movement and impact
Unit cost = comparable net cost ÷ comparable quantity
Change % = (later unit cost − earlier unit cost) ÷ earlier unit cost
Invoice impact = unit-cost change × later quantity
6. Verify exceptions
Review increases, decreases, new items, and missing items. Attach both source lines and state the exact question.
Worked comparison example
| Item | Invoice A | Invoice B | Result |
|---|---|---|---|
| Sunflower oil | $270 ÷ 60 L = $4.50/L | $288 ÷ 60 L = $4.80/L | +$0.30/L · +6.67% |
| Bread flour | $140 ÷ 100 kg = $1.40/kg | $146 ÷ 100 kg = $1.46/kg | +$0.06/kg · +4.29% |
| Paper cups | $76 ÷ 1,000 = $0.076 | $74 ÷ 1,000 = $0.074 | −$0.002 · −2.63% |
The oil increase costs $18 on the later invoice. If you buy 180 L monthly, its forward monthly impact is $54. The spreadsheet shows both so a one-time invoice decision is not confused with a forecast.
Common false positives
- A 12-pack became a 10-pack.
- Freight moved from a separate line into product price.
- A discount or rebate appears only on one invoice.
- Currency changed or the exchange rate was applied inconsistently.
- A credit note corrected the earlier invoice.
- The later line is a substitute, not the same product.
Frequently asked questions
Can I compare invoices from different suppliers?
Yes, if the items, specifications, units, service levels, and cost basis are genuinely comparable. Record differences such as minimum order, lead time, freight, rebates, and quality separately.
How should freight be treated?
Use one documented rule. Allocate recurring freight if it is part of landed cost; separate one-time or route-specific charges when they would distort unit price. Apply the same basis to both invoices.
What if there is no stable SKU?
Create an internal Product ID and maintain a mapping table from each supplier’s SKU and description. Keep ambiguous matches for manual review.