A rain check is a guarantee that an on-sale item that is out of stock can be purchased by a customer at a later date at the sale price. Doing so keeps the customer from going elsewhere to make a purchase from a competitor. A rain check policy is needed when a business underestimates the number of units it will sell, and so stocks an insufficient quantity to meet demand. The evidence for a rain check is usually a paper voucher that the customer must present when buying the item at a later date.
If a business wants to avoid issuing rain checks, it can advertise that supplies of sale items are limited, thereby inferring that once supplies run out, no additional units will be sold at the sale price.