Selling here, I only charge however much AusPost charges me for shipping! I also have quite alot of boxes and bubble wraps that I can re-use from previous buys, and of course I don't charge for the scotch tape or paper to write your address and neither do I charge for the labour!

But when selling on eBay, I usually add 1-2cm more on the box dimensions just to cover me in case for any extra charges due to actual shipping weight of item.