According to the FTC's Retail Food Store Rule, grocers must 
offer rainchecks or product substitutes of comparable value 
when they run out of advertised items. They also can comply 
by ordering quantities of the item sufficient to meet 
reasonably anticipated demand or by disclosing in ads that 
items are available only in limited quantities or only at 
some stores. Although the specific terms of the Rule apply 
only to retail food stores, other companies advertising 
products available in limited quantity or only at some 
stores may want to make similar disclosures to reduce the 
risk of deception.