When it came down to it, it turned out that the bulk of the envelopes contained marked up photocopies and printouts with some printed proofs, so I was able to distill the returnable pile down to a fraction of the original stack.
Difficult to do, though. If I have problems recognising the garbageness of ... well ... actual garbage, these printouts (that had genuine value and conveyed real meaning several years ago when I did the work) were difficult to redefine as rubbish. Besides, some of them were very pretty. I could hear a voice in my head warming up to provide a defence of them, so I thought it best to rush them to the recycling before I changed my mind. I still sorted through them three times to make sure there wasn't any real artwork in there.
I kept them for up to seven years, and they've not been called for in that time, so I think it's fair to say they'll not be needed again.
