dextrous on "[Plugin: WooCommerce - excelling eCommerce] Quantity Discount Across Product Variants"

I'm looking for an extension that will apply quantity discounts across sizes for an item of clothing.

An example of how this works can be found here:
https://www.uniforms.com.au/Shirts/418jk-Signature-Corporate-Striped-Shirt.html
where the number of shirts ordered across all sizes is totaled, and the discount applied at the checkout.

Does anyone know of an extension that has this functionality?

Thanks

http://wordpress.org/extend/plugins/woocommerce/