Here's how it works: Frenchephaistos gets hold of a suitable box, fills it with goodies, and mails it to the next in line. Let's say AndyAaron gets it. He looks over the contents and decides what he thinks is cool. When I had the boxes making their way across the States, it was understood that you had to put stuff in if you removed more than one item. (I put at least one item in for every item I took out, sometimes more.) Andy AndyAaron would then repack the box and send it to the next MTo'er. The Stateside box kept growing in size. It's pretty cool to see how it grows, and you have to wonder who claimed the stuff you put in...
Yes, you put in as much as you take out, in value at least. You can for example take two 20€ items and put a 40€ item in its place, or the other way around. The real reason behind this is to give away things we don't use anymore but that we think will benefit someone else on this great forum. When you get the box, you get to keep what you like, but also play with a lot of things you've only seen online, but in person. It's a bit like an EDC Christmas for adults.