In all fairness to RM, I don't think they could give a stuff about how much the item cost. If you had sold a diamond for a couple of thousand but it fit into a standard size envelope (ignoring insurance) and were proposing to post it ordinary post (stupid I know but this is hypothetical!)...
We received loads of cards in small envelopes and never paid a fee. Surely all you need to do is go to the sorting office, ask for them to show you how it was underpaid and when they show you the item, decide if it is worth the fee! If not, walk away.