I have completed quite a large number of trades for CMFs on here, probably at least 60 now. I've noticed that the ones I get sent are often sent as parcels rather than large letters, thus cost significantly more to post for the sender. So I thought I'd share what I do when I pack them. I know some people do exactly what I do as I often receive them packed like this too, and I learnt this from someone I traded with. I cannot remember who it was, it was so long ago.
And in case anyone thinks I am a tightwad - I am, so that's out of the way.
Anyway, the reason for this is that a large letter (up to 100g) costs 90p to send first class, a packet (up to 750g) costs £2.70. Second class, the prices are 69p and £2.20. I don't know about postage limits in other countries, but you might be able to adapt it if you have large letters vs parcels too.
If you are sending six minifigs, then they are normally under 100g, so it is clearly better to send them as large letter. What I normally do is tape each packet closed, or put them in a small bag. It's best to keep it as small as possible (or pack it and fold it and tape it) so that the pieces don't move around to much. The trick is that you then tape the packets together side by side or tape them to a piece of paper next to each other. When they go inside a jiffy bag, this ensures that they do not shift about and so the envelope does not become any fatter if it is shaken / dropped / held upright as the packs don't all end up at one end. If the envelope is under 25mm and fits into the postal guide for large letters it qualifies for large letter rate rather than parcel rate. If the minifig has a large utensil / headgear (Aztec!) then it is often an idea to remove this from the packet and bag it separately (again taped to neighbouring packs or the piece of paper), and I sometimes remove the black plates too and bag them separately, or stack them. Capes usually go inside the leaflet, or get packed against a piece of card, so they do not get crushed when you tape the packet closed, especially if you fold the top down. This all helps keep the width of the packet down. If you can keep it to under an inch, you will save money.
Especially if you are only trading a few minifigs, the saving can be £1.50 or £1.80. If you do a few trades per week, this soon mounts up so is worthwhile trying it out. I find it takes virtually no time to do. I nearly always post mine from a post office, so you can quickly check if it fits though the guide, and get proof of posting at the same time. If anything goes wrong I'd always refund the other trader the Royal Mail fees for getting the postage wrong - but it never has so far. Plus you have proof that the Post Office checked it fit through the guide, since they gave you a proof of posting when they took it.
I also recycle jiffy bags - often ones from lego. Again, it's nothing original and I know other people do this, since some that I have received I can trace to at least three previous bricksetters by the address labels! One once started with me (reused from a lego PAB online order), got sent to two other bricksetters and got returned to me in another trade. And if you use bricklink, chances are you have loads of small resealable bags.
I hope this helps someone save a little bit of cash. They way I think about it - every time you do it, you can afford another CMF.