Miaow, I actually made my own thickness measurerer out of cardboard lol .5 cm is regular letter mail, 2cm and under is oversized letter mail up to 500g. I just measured it all out on a piece of card board and cut the hole out with a box cutter and voila. I did this shortly after paying more to mail an envelope then a giant 1kg box a few days prior, I had no choice but to mail it out, so I had to pay. I complained and they did absolutely nothing about it -.- weather I was wrong and it should have been small packets, an envelope should not cost anywhere near the same to mail as a large box. But I learned its best to get everything as flat as possible. If the thickness is more then 2cm, put it in a bigger envelope. The size of the envelope doesn't really matter with oversized mail, it's weight and thickness.