Parcels under 500g all go at the same flat rate of $7.45 to anywhere in the country. Once they get over 500g, they are cubed and the cost is the cubed weight, not the actual weight. If it's a smallish item, often it works out cheaper to send in a 3kg or 5kg satchel. You would still pack into a box and then put the box in the satchel.
A couple of weeks ago I posted an order to Qld. The packed weight was around 1300g. If I had sent it as it was, it was going to cost close to $30. I ended up posting it in a 5kg satchel which was less than $20. Thankfully most of my stuff goes either large letter for $1.40 or small parcel. It's only when people buy multiple items that it goes over 500g.
As for packaging materials, most of the time it is cheaper to buy it on eBay. First you need to work out what you are going to need. That will depend on whether you are going to sell fragile breakable items, or new sealed in box items, or other unbreakable, unboxed things like clothing. It is also far more economical to buy it bulk rather than as you go. That goes for boxes too if you need to buy them.
If you're going to sell new sealed in box items, then apart from boxes (which you can get from supermarkets, Bunnings, the local hospital if you know someone who works there), all you'd really need would be packing peanuts. If you're going to sell say clothing, then you'd really only need some sheets of tissue paper that you'd wrap the clothing in before putting it in a satchel. Even then, some people don't even bother with that, they just put it straight in the satchel.
If you're going to sell fragile stuff, then your packaging expenses will be much higher. You'd need bubble wrap, packing peanuts, maybe tissue paper depending on what the items were, and that thin plastic coated foamy stuff (I have NO idea what that's called!). You can use newspaper to pad out boxes, but it does add a lot of weight. If they are really low weight items, but need to be packed well, then newspaper would be OK.
You should also buy some digital kitchen scales (mine were $19 from K-Mart a few years ago and weigh exactly the same as the calibrated scales at the PO). If you're going to be sending in boxes, you'll need rolls of the thick packing sticky tape. Regular sticky tape is quite handy too.
Check out how much the packaging items cost on eBay, then go and check out prices at the local Officeworks if you have one. Sometimes it does work out cheaper to get them from there when you take postage costs into consideration.
Thankfully I don't pay for any packaging materials. I recycle what I can from items I have bought and I have friends that work at the local hospital who get me lots of stuff. If I did have to buy everything, I'd be adding a few extra cents onto my postage costs or onto the item price to cover that stuff.
