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For non-collectors the easiest way to kill flies is in the freezer - just put them in for 2 hours and when they're dead handle them very carefully (they will be brittle) and let them thaw a little.

I think the easiest way to send flies is in alcohol (isopropyl or industrial methylated spirit) but the main thing to remember is that is must be pure - with no colours or additives that might stain or contaminate the specimens. Use glass or plastic vials with very tight-fitting stoppers and pack-out the specimens in the tube with a little tissue paper (not cotton wool - it gets tangled in the bristles & legs) to stop them sliding about too much. Then just wrap the vials in bubble-wrap or cotton wool and place in a strong, cardboard box for posting. The resulting specimens usually won't be perfect but most experts will be able to extract and mount them themselves.

If you feel that you have time to pin specimens then it will usually make them easier for the expert to handle and will make for better quality specimens. You'd just have to get hold of some pins (from a place like http://www.entosp...) - either fine (diameter depends on size of fly) 38mm stainless steel (not black) or fine micro-pins. Then (while the specimen is fresh & still soft) just pin the fly to a piece of dense foam or cork and leave it for a few days to dry. The flies can then be pinned into a foam/cork-lined cardboard box or even film cannister - just make sure the pin is firmly in the foam. Then to protect it in transit (very important) put this box inside a larger cardboard box and pack it with bubble wrap or foam so it doesn't rattle about. Mark the box "fragile" on the outside (just in case it makes any difference!) and post it using a relatively fast delivery tarrif.

Easy :D