A coupon limited to one use (Number Available = 1) is used up as soon as the Payment Details page loads, before the customer pays. If that page loads again, from a refresh, going Back, or a declined card, checkout rejects the code as "not found, has expired or is not applicable" and charges full price. On our store, about 4 in 10 single-use codes that got used up never produced a discount on a paid order.