When a business advertises specials and they run out of stock, they often have a policy where they will honour the advertised price at a later time--when the sale item is in stock. This involves them writing out a slip of paper with the details of the sale item and the quantity you wish to purchase and then when they are stocking the item and you present the slip to the cashier, and they will give you the item at the sale price, even though the offer period has expired.
It is a great thing to remember that you don't always have to walk away empty-handed when you rush out to your local supermarket only to find out they have sold out of the item you were planning to buy. It is not a common practice here in Australia to ask for a raincheck, so don't be surprised to see a few blank looks from the customer service staff when you ask for one, but it is well worth asking. What's the worst that can happen? They say no?