If a customer got a bigger discount than you expected, check the coupons on the order. In almost every case the coupon is working as set up, but one setting gave more than intended. Check these, in order.
1. Applies to Already Discounted Prices is set to Yes
With Yes, the coupon comes off prices that are already discounted, so the customer gets both discounts. A club member gets their club discount plus the coupon. A customer buying 6 gets the 6-bottle price plus the coupon.
Example: a $28 Rosé, 15% club discount ($23.80) and a 10% coupon: with Yes the club member pays $21.42. Six Shiraz at $195 for 6 with a 10% coupon: $175.50.
Fix: open the coupon and set Applies to Already Discounted Prices to No, unless stacking is what you want. See What does Applies to Already Discounted Prices do on a coupon?
2. The coupon is set to Auto apply
With Auto apply set to Yes, every order that meets the coupon's conditions gets the discount, without anyone entering the code. It applies to every customer and channel the coupon allows, until the coupon ends.
Fix: set Auto apply to No, or limit the coupon's Channel and Customer Pricing Group, and give it an end date. See How do auto apply and combining coupons work?
3. Coupons are allowed to combine
With Allow combining with other coupons set to Yes on every coupon used, the customer gets all of them, for example free Rosé plus free shipping plus 10% off.
Fix: set Allow combining with other coupons to No on coupons that shouldn't stack.
4. A Fixed price coupon has no products chosen
A Fixed price coupon with no products chosen goes on the most expensive products in the cart.
Example: a "$20 for 1 item" coupon turns a $110 magnum into $20.
Fix: choose the products in Required Products in Order or Applies To Products. See What are the coupon discount types?
5. A percentage discount on a free item applies to every one
A 100% Percentage discount on Different Selection products makes every one of those products free. A Percentage discount has no limit on how many products it applies to.
Example: "buy 6 Shiraz, Rosé 100% off": a customer who adds 3 Rosé gets all 3 free.
Fix: use Fixed price with Fixed Price Amount 0 and Fixed Price Quantity 1 to give only one free bottle. See How to make a coupon that only works with certain products
6. An old coupon is still running
A coupon with no end date and no redemption limit keeps working. Old codes get shared online.
Fix: give every coupon an Active until date or a Redemption count limit. See How to make a coupon stop automatically
How to check your coupons quickly
In Settings > Coupons, the list shows Applies to Discounted Prices, Auto Apply and Status for every coupon. Filter by Active to see what's running now.