Ah, Paul, I feel the pain RE that poster.
Common sense would have been to contact you and say "Hey, look, I know I said postage would be this much, but could you send a little more as it is going to cost more than I thought to send".
Postage isn't cheap now for rolled posters in the UK.
Small parcels are automatically £5.20 and that's before adding a signature and the cost of the packaging.
I think a signature is now £0.95 so that's £6.15, then the cost of a very good quality tube is around £1.60 in bulk (around £3.00 when bought separately) so there's a minimum of £7.75 and then for some there is the cost of actually travelling to the Post Office.