The extra 12c is a fee charged because the item is "non-machinable", which means it won't be handled well by the high speed mail movers. Unfortunately, it doesn't guarantee that some employee won't try to sort it that way. Which is why it's best not to us an envelope that looks like it "might" go through the sorter OK. They know what bubble pack envelopes look like and will seldom try to auto-sort these. With thick envelopes, I've found that even with 49c postage, they sometimes end up in the stack headed for the sorter. Marking them "Hand Cancel" sometimes helps, but it's best to not use an envelope that looks like a letter at all.