When I, my mom or my grandmother made jelly/jam/preserves, we always just used a layer of melted parafin wax. And we didn't use canning jars either. We saved appropriate-sized jars (from pickles, peanut butter, etc.) and used those for jelly. We all had a separate pan (or more likely an empty coffee can or something similar) for melting the wax. As the jelly cooled, the wax cooled and sealed the jar. I'm aware that that method is not advised nowadays, but it's been used for decades. None of us ever had a problem and the product would last for years. We never refrigerated our open jelly jars, either. My family went through it so fast it never had a chance to get moldy or whatever.