This may seem a bit obvious, but if you're trading with someone, there's not usually foam trays involved, what works best for you? Tissue, packing peanuts, individual wrappings?

Individually wrap them in bubble wrap, and then fill whatever box your sending with a material such that they don't bang or slide around (tissue or paper towel work just fine).

I useley use bubble wrap or toilet paper works great

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Moria: 185 units
Durin's Folk: 181 units

If its just a few figures then I use a padded envelope. I wrap each figure individually with bubble wrap before putting them in the envelope.