I only just checked my feedback and noticed a comment: "The postage paid by the other user was insufficient . Comment: 2.20 in cost due to lack of postage" Now I thought the RM charged you £1 to pick up items with insufficient postage plus the extra postage missing, if this were the case then...

The postal charges as stated by RiSi in the Swap Details are hopelessly out of date, and maybe this person didn't realise that. The other possibility is that s/he thought it would go as a 'large letter', and RM deemed it to be a 'packet'. Even the slimmest book can measure more than 25mm thick once it...