I had an eBay purchase recently where the seller thought the jiffy bag would qualify as a large letter, but it clearly wasn't. I got a card through the door telling me that the postage had been underpaid and I would have to login to RM with the card ID and pay the additional postage, and the item would be delivered 2 days later. This all went smoothly and I agreeed with the seller that he would pay the extra postage. I would imagine there would be something similar for the transition period of Peak boxes, but Peak would presumably then charge the customer? I could see where RM might collect a few and charge them once a week, rather than daily, so maybe that's where the film is?