I'm in a similar situation to those who have not received there stickers above, however a mystery A4 brown envelope as posted by Coleman is waiting for me to pick up at the sorting office due to underpaid postage.
This would appear to be the problem, as he has sent first class, not first class large letter.
I think this is a genuine mistake chaps, and him being friends with captainlip has drained down his mental capabilities, so just be patient and I'm sure it'll all get sorted...
(and he is now aware of what the problem is, so should be sorted from now on)