The invoice diff, not the price list
Suppliers keep two prices: the price list they show new customers, and the price they actually invoice. The gap is where the creep hides. Every invoice needs a line-by-line comparison against the last invoice for the same SKU, not against the quoted price list. Automation makes this trivial; manual spot-checks catch maybe 20% of it.
The 3-invoice trigger
Any SKU that rises on three consecutive invoices without a written notice from the supplier triggers a conversation. Not every rise is unreasonable — currency, harvest, minimum-wage impacts are real — but every rise deserves an explanation. Suppliers who cannot explain a rise usually reverse it on request, because they know the next call is to a competitor.
The quarterly reset
Book a quarterly 15-minute review with each major supplier. Bring the invoice-diff report. Ask three questions: which SKUs moved, why, and what's coming next quarter. Suppliers respect operators who look at the numbers; they exploit operators who don't. The review costs an hour a quarter and typically pays back 2–3 GP points annually.
Diff invoices, trigger conversations at three rises, review quarterly. Price creep is a habit, and the fix is a counter-habit.
