What do rain checks allow customers to do?

Rain checks are designed to help customers secure the opportunity to purchase out-of-stock items at the promotional sale price when those items become available again. This practice ensures that customers do not miss out on special deals due to inventory shortages. For instance, if a store runs a sale on a popular product that sells out quickly, a customer can obtain a rain check allowing them to buy that product at the sale price once it is back in stock. This approach maintains customer satisfaction and loyalty, as it upholds the promise of the sale even if the item is temporarily unavailable.

The other options do not align with the main function of rain checks. Reserving items at a discounted price and returning items for a full refund are different policies that do not relate to the availability of out-of-stock items. Similarly, reserving a spot in a cooking class is unrelated to in-store product promotions and inventory management. Therefore, the correct answer directly correlates with the purpose of rain checks within retail practices.