Hi,
I'm with Dave-S on this: the Royal Mail introduced this scheme, partly because of the hassle they had with shipping vast quantities of the last "Harry Potter" hardback book, via Amazon and Tesco's online outlets! It makes more sense to me, that something large (but light) like a giant birthday card, or a bunch of helium balloons in a box, should be charged at a lower rate, than a very heavy hardback book.
And, as others have said, see where else you can get service whereby your parcel will be sent relatively quickly at a relatively low price. I'd rather use someone like the Royal Mail than any number of new, fly-by-night companies who think they will ever beat the RM at their own game!
Simply put, like every other British institution, the RM do the best they can, with the resources they have, whilst the Government do everything they can to screw it up (by closing lots of small branches down, and then installing Post Office sections in larger branches of WH Smiths)!
As far as I'm concerned, the Royal Mail are pretty good at what they do, despite their occasional flaws and errors.
Pooch