Is It Safe To Store Coins In Modular Stacker Containers?

PETE, Polyethylene, Polypropylene, and Polystyrene are usually safe for use. With that being said, low quality plastic can have impurities in them that could not be ideal over long periods of time, and you are not really going to be able to tell if that's the case or not. If you look at the bottom the plastic type will be labelled, or there will at least be a recycling code.

The only common plastic type you need to outright avoid is PVC. There are some other uncommon or specialized plastics that could cause problems, that I'd doubt you'd find at Walmart.

http://en.wikipedia.org/wiki/Resin_...ication_code

However, remember that every time you open it, new air and new moisture enter that container. And it is fairly air tight so constant new moisture would mean constant additions of moisture absorbing substances.