The first thing you want to avoid when storing coins is proximity with anything chemical that would cause the coins to change their character. For example, sulfur is used in the process of making many types of paper. If you put coins in a paper envelope, make sure it's an acid free and sulfur free envelope. A plastic flip is very common in the coin collecting field. It has little envelopes that fall in the middle and you usually put a little cardboard insert that describes the coin in the other pocket. That way you can pick it up in your hand and read with the coin as to look at coin on both sides. It is a handy little device. These flips are made in two types. One is a PVC product and the other one is what is called a safety flip. It has no PVC in it. If a coin is left exposed to PVC long enough it will cause a change in the surface condition of the coin. I have personally had coins in PVC flips for several years and had no problem with them, but I think for the long-term, it would be smarter to put them into either an acid free paper envelope or a safety flip.
