Hooks tend to damage envelopes and sorting machines can damage flies or be jammed by them. I put flies in some kind of hard container ( altoids tin, empty bullion cube jar, etc.) in an envelope. Insurance is extra and I don't bother with it. The rates are high and the odds favor just planning on replacing a set of flies if necessary. I've never had to do it yet but some people in a swap I ran did have their flies lost.