Just imagine the chaos this would bring to Singapore. Every Tom, Dick and Harry would run around after purchase for 7 days trying to find a cheaper offer and get the price refunded, wasting petrol, his and other people's time in the course of saving 10 bucks... Deal is deal.
Even if some shops put up these sings and offering refund, they won't make it easy for anybody to find something that matches the criteria. It's marketing, after all.
Because its simple and a simple thing can be done to solve it simply but you went to post in a simple forum so we simply gave you out two cents in as simple a manner as possible. Do a simple search please.