International shipments or packages under 1kg don't have their dimensions considered.

For domestic over 1kg, the cubic weight formula is:

Cubic Weight = L x W x H (in meters) x 250 = Weight in kg.

So, if you have a 20cm x 20xm x 30cm box that weighs 1kg, the cubic weight is:

.2 x .2 x .3 x 250 = 3 kg, so you'd be charged the 3kg - 4kg rate.

Now, in this example, if you could shave off 1cm of any dimension, you'd get:

.2 x .2 x .29 x 250 = 2.9 kg, so you'd be charged the 2kg - 3kg rate.

So basically you just gotta do the math to see if a cm or 2 will make a difference.