When u go to stores/supermarket and cannot find the items that are on special, u can ask for a raincheck at the Customer Service counter. Raincheck is a small piece of paper that let you pick up the item, at the advertised lower price, within 1 month (most stores) to 1 year (Zeller).
海报上写着no Raincheck = while supply last = when they run out of the item, no more (cheap price).