People seem to get pissed when you pay/charge £3-90 for postage and the actual price RM charge is £3-90, the box/packaging and brown paper if used is not included, so £5 is fair enough, so they see £3-90 on stamp and think "I paid £5 .... I'll give 3 stars for postage"
Postal charges are advertised so if you don't like it don't buy it... Simple. RM seem OK here as do Hermes, yodel not so.