The major things I though were:

The price doesn't add up

They sent the bill to the receiver

They didn't frank the envelope

They are idiots

To answere some of the stuff people said:

This is a card envolope and not cheap, and less than 1/2 millimeter thick, I am not trying to go cheapo or anything. Postage I find is the hardest thing to work out when selling and unless this item got hit with 20kg of feathers I think it would be OK.

If 20kg did fall on it, it would be quite flexible. The components are held in place. I have sent these about 50 times, if anything they should pick up my bubble mailers that if shaken force everything to one end.

A customer of mine payed an incorrect fee for some reason to get their item. Thankfully they were quite good about it after I refunded them. Maybe it was a scam...,