I guess each country is different. As far as I am aware, a shortage of postage to something sent here would end up with the recipient being asked to pay the extra (plus a charge of course!) but I only know that for sure for domestic postage.

So far it sounds as though I have been lucky, but I have never had anything undelivered because of a shortfall on the postage. I am not sure that the Royal Mail here would know anyway what the correct postage for something sent from the USA would be?

God forbid though that they would make the system simple - I always read with interest the troubles that people seem to have with the USPS. Our Royal Mail may not be perfect but it does seem to be simpler