Customise Product Options for shipping

Hi all,
I am using 1.5.2.1 version of opencart.
I decided to just use the options for the postage for my products.
The reason for this is so the customer can view the postage before having to go to the payment page first
But the problem is:

We sell curtains
Shipping is $10 for 1 or 2 sets
$20 for 3 or 4 sets
$30 for 5 or 6 sets
and so on....

What would be the easiest way to set this up please anyone?
our website is www.expandatrack.co.nz

You could use the built-in Weight Based Shipping, and set the weight of all curtains to 1. You'd then create a rate like this:

2:10, 4:20, 6:30, etc.

Or, if you are using the weights for something else, you could use Item-Based Shipping.

hi thanks for that. I may do that,
Is there any way i could edit the files so i could do it with the options?

Options can have their own weight, so you should be able to adjust their weights to fit your rates accordingly.