Looking for way to use royal mail post sizes on PS 1.4.8

[BlizzardUK]

Just found a problem with my site, I need to list a mug which is big but not heavy, I currently have the website in weight for postage and sell worldwide. So because the item is bulky but not heavy it fits in to the Small Packet (SP) range instead of Large Letter (LL). So if I change the weight to make the postage right for the UK (shipment base) it becomes totally off for either Europe or Rest of the World. I need a module that allows me to change a product postage to LL or SP but using the same weight. Things were fine before Royal Mail changed things a year or so back to dimensions instead of just weight.

 

Any help or ideas ? Thanks.