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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.