Postage Problem that I cannot solve- Big Royal Mail

Currently have installed the Big Royal Mail Postage Addon.

My client sells car light bulbs in two different pack sizes:

A single bulb parcel weight 125 grams
A box of 10 bulbs parcel weight 250 grams

At the moment we are trying two different methods to achieve this:

The first, the prefered method, using attributes setting a price for a box of 10. The problem here is that Zen Cart calculates the shipping weight as 1.25 kg charging the customer too much money.

Is there a way to set a ceiling on the actual shipping weight or the amount charged through the attributes or other section in the admin panel?

The other method, not prefered as it is a bit long winded for the client, is to have two different listings for the bulbs, selling a single bulb on one listing and a box of 10 on a different listing.

Any help or suggestions would be gratefully appreciated.