[freosam]

Oh right, I've never seen polythene resealable ones here in Australia. (The floppier 'freezer bags' are I think, but they're less useful for archiving.)

I think it's mainly PVC that's to be avoided for archiving, and office supply shops are full of the stuff (document sleeves, etc.).

An archivist once told me that if you burn a bit of plastic and it doesn't give off any smoke then it's likely polypropylene or similar, and so good to use. That's never felt like a particularly robust test though (but I'm not a chemist).