Sorry to hear about your problem with Royal Mail Berniece, I've had two letters recently that didn't have the correct postage and I paid on line and they were delivered OK.
I do wonder about the way the post office check size etc. I took a card in and the man pushed it through a slot and said it would be normal postage. Next time I took one in I was told it would need a large stamp. Both were the same type of decoupage cards. Don't know how you can guarantee the postage is correct. Anyone got any ideas?