Quick question - I have this installed and it's working great on my cart (thanks a lot!), but I'm having a slight problem with letter sized objects. The option is appearing, but the price isn't affected by the weight of the object.

For example, a letter from Australia to the US weighing less than 50g is calculating at $2.05 which is correct, but when the weight is increased to 250g it is still calculating $2.05, when it should be $6.15.

Is this a known issue and is it fixable? It's important as most the items in my store will be posted as letters rather than parcels (which do calculate correctly at different weights).