Well, a month down the line still no nearer a solution. Abantecart have done very little to help. It seems the problem has been ongoing and stems from the fact Royal Mail charges by weight, but in distinct size categories. For example. a small parcel of 100 grams is a completely different price to a large letter of 100 grams.
Abantecart's Royal Mail extension is entirely weight based so cant distinguish different size categories.
In february they said they would put it on their todo list
http://forum.abantecart.com/index.php/topic,611.0.htmlBut as it is. it is useless. Is anybody working on updating it?