Sellers always explore ways to keep you postage costs down. I have been charged $5.00 for postage when the item could easily have been posted for $1-$2.
Small items of clothing eg. singlets can be folded in half and placed in a snap lock bag to keep them flat, then posted in an A4 envelope.
Be creative, Re-cycle. Keep satchels turn inside out and reuse. Great for childrens books.
The Australian Post web site has a great postage calculator, weigh your item before listing, check it's dimensions and use this calculator to help you decided the cheapest way to post.
Remember to make sure that your item is packaged safely, use bubble wrap if needed.
Buyers will be willing to pay more for your item if the postage is not too expensive. Don't rip your buyers off by trying to make money from postage, cover your costs but remember postage is exactly that, POSTAGE. Not a way to try to pull a few extra dollars from your buyer.