My experience is that they don't compost, at least not in the timescale that I use my compost. And shredded plastic is almost impossible to remove! So I bin the envelopes. Sellotape doesn't compost either, so best to remove it from cardboard boxes before they go on the heap, but at least that's easier to remove from the compost (I keep a bin bag next to the heap for all the things that get in there and shouldn't have.)

I don't bother to shred cardboard - sheets of it always have huge numbers of worms under them. I shred paper if required for security reasons but not to aid composting.