You may want to experiment a little with eBay's rate table option, because you can set up postage costs and discounts according to weights. It's especially useful for sellers who use couriers and want to specify different costs to different locations, but also once you select to use rate tables, you get the option to set up blocks on buyers by region (often very useful for plant sellers who are not able to send to all areas of Australia).
http://pages.ebay.com.au/help/pay/shipping-costs.html#tables
The other alternative is combined postage discounts, as you can account for reductions in some costs either by a specified dollar value, or percentage. Those apply automatically in about 75% of circumstances, it just depends a little on how the buyer purchases and what device they're using (note, the discount rule has to be applied to all listings that you want it to affect, and the postage service selected has to be identical, so if you have one that uses "standard postage" as the default option, and another that uses "Australia Post Parcel Post", the discount won't automatically apply and a manual invoice will still be required).
