We take your coupon offline when the budget is 80% utilized, and we allocate the remaining 20% to cover for possible redemptions from customers who have already clipped the coupon. For customers to have the best experience with coupons, we allow them to redeem coupons for a window of approximately 30 minutes after we disable the discovery of a coupon.
If 20% of your budget doesn't cover the customer redemptions in this 30 minute window, you might see your spend exceed your budget. The risk of your spend overshooting your budget decreases for higher budget amounts. Coupon budgets are for planning purposes only, and any level of overshoot should be expected if there is a mismatch between the duration of the coupon, the depth of the discount, the budget, and the pre-coupon sales velocity of the product.
To learn more, refer to How to set up efficient budgets.