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Yep, seems only pinned or "alcoholized" flies will do. I wish there was an easier way, but there isn't. Pity about that large collection.

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Yep; I was really gutted to see the state of that big collection. So sad to see damage on such a vast scale. The collector probably spent hundreds of field hours making the collection only to destroy it through bad trechnique.
Incidentally, some museums seem to have concluded that there is no safe way of sending flies in the mail and now insist on specimens being carried by hand.
We can only do the best we can
cheers
Adrian

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It is possible to send pinned flies safely through the mail - if they are properly prepared (that's a whole new thread) and if they are sent using one of the more secure systems. Sending alcohol material in the mail does require special training (at least in Britain and the USA) and the use of selected couriers.
Even taking stuff by hand can have its dangers - I know of one instance when a bag containing storeboxes of pinned ichneumons broke while the entomologist was on a moving escalator. He watched helplessly as the storeboxes tumbled down the steps!
No, it wasn't me! )

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As a follow-up to Irwin's comment. I just received a box of pinned tabanids, small and large specimens, all the way from Europe (I'm in Canada). The trip took 3 weeks. Absolutely no damage, not even a leg or antenna broken!
The trick seems to be to in the packaging; in this instance: a 19x10x4 cm unit tray with a double layer of foam on the bottom to hold the pins firmly. This tray, with a lid, in a much larger box 30x26x21 cm. Surround the tray with foam 'peanuts' so that it can't move.

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Often it is recommended to "cross-pin" the insects before posting them. This involves pushing a sloping pin on either side of the abdomen (for large, direct-pinned specimens) or mounting stage (for staged flies). This will stop the specimen swivelling on the pin, so it will not come loose from the foam, nor will it knock into adjacent specimens. Card mounts are particularly prone to turn round on their pins and can cause alot of damage by hitting specimens that are close by.
It's also a good idea to pin some cotton wool along one edge of the tray - if any pins or specimens do come loose, they get tangled in it and will not damage other specimens.
It's also a good idea to seal the tray with a temporary lid made of clear plastic film. The allows customs officials to see inside the box, if they get suspicious, without any risk to the specimens. It also acts as a barrier to pests that might want to get at your specimens. It all takes time, but it is worth it to ensure that your specimens arrive in perfect condition.

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Because I've been studying part time and remotely the last couple of years, I have posted a number of boxes of specimens off to tutors to be marked. I would say always cross pin in the manner Tony describes. My experience is that the single biggest danger to your specimens is a card swinging around and wiping out the specimen next to it.

Tony's ideas about some cotton wool to tangle up loose stuff and the cling film seal I hadn't thought of, but they make sense to me and I will be adopting.

Lots of bubble wrap around the box, then posted in a padded bag worked fine for me, but I was only sending one box of about 18cm x 12 cm x 8cm within the UK.