I am going to begin to bake my Madeleine Cooke's and sell them at my local farmers markets. They are very moist more like a little cake. I heat seal them in a heavy plastic bag and then put them in my box. They taste great after 3 weeks but are not moist any more and that is very important to the cookie. Is there a natural or "not bad" preservative that I can purchase to keep them fresh and moist? Please help, these cookies are fabulous and I know they can be a wonderful addition to cookies!