You set a 30% sitewide sale. A shopper applies a 'WELCOME10' code at checkout. Now they're getting 40% off, your margin is thinner than you planned, and you may not even notice until you reconcile the month. Multiply that across a busy weekend and discount stacking becomes a real, invisible cost.
Why it happens
Most stores accumulate discount codes over time - welcome offers, influencer codes, win-back campaigns. They're easy to forget about, and they don't automatically switch off when you launch a sale. So during your biggest promotions, when traffic is highest, they quietly combine with your sale prices.
The real cost
The damage isn't just the extra percentage. It's that your advertised price is no longer your floor, which makes forecasting unreliable and can turn a 'successful' sale into a barely-profitable one. Worse, savvy shoppers share stackable combinations, and the leak spreads.
Your sale price should be the lowest price. If a code can go underneath it, you've lost control of your own promotion.
The fix
Block other discount codes from applying while a sale is running. With stacking turned off, your sale price holds as the genuine floor - shoppers still get the deal you advertised, and you keep the margin you planned for. When the sale ends, your codes can resume as normal.
It's a small setting with an outsized effect: predictable margins, honest pricing, and no end-of-month surprises.