Royal Mail shippping - limiting services by size

Hi

I've got an interesting problem and Im struggling to find a resolution for it
I sell a number of products that are all different weights and sizes, can be large and light or small and heavy
Im sending it all by royal mail at the moment and using the built in royal mail shipping extension (im on opencart 1.5.6.1)

I have a situation where I have a light item that is too big for standard post, yet standard post is still being offered
I've set the dimension on the product to a size that I would expect it to be only offering a parcel (120,60,60cm), yet it still offeres first class post alongside parcels, obviously the customer is going to choose the cheapest delivery method and I end up loosing money or having to contact the buyer and explain theres been an error (not somthing i want to do)

any advice is welcomed
Thanks
Phil
www.makeitbuildit.co.uk

I believe the built-in Royal Mail just uses weight to determine its rates. I've had a number of people in the UK use my Formula-Based Shipping for Royal Mail rates, they just set them up based on the box size dimensions and eligible weights.

Note that it doesn't use a 3-dimensional packing algorithm, it just checks whether each item is within the dimension limits, and whether the overall volume would fit in the box. But it works pretty well, and definitely better for Royal Mail than the built-in extension.

Feel free to take a look at the screenshots and demo site, and if you're interested let me know at www.getclearthinking.com/contact if you have any further questions.