You know if previous buyers were happy with their purchases came back and saw lower prices, they just may say "hey great, now I can buy another!" No, I wouldn't refund. They paid what they thought was a fair price and they got their stuff and are presumably happy with it. Unless the price cut is drastic, they probably won't realize it anyway.
But I do have to agree with whomever said, are you sure you want to do this anyway? If it's only a little change, and your items have been selling, you don't really need to lower prices every time you save a little on costs - accept it as a savings in cost for you and use it as profit to grow your business - advertising, new packaging, whatever. Besides, without any cushion, what if your materials costs go up - are you going to raise them again? You don't want to go back and forth too many times.