i had this same problem last week at my local gas station.. i filled up my car almost 45$ worth of gas, the next day i drove by the place and gas had went down nearly 10c..
also last month, i got a few dress shirts from my local suit store. the next week they went on buy one get 2 free..

of course when i went in , they wouldn't give me my money back..

so um.. its common practice for business to not refund when things go on sale.