So I put stamps on the envelope I send you? And that's all I need to do? Will it still be the same since they would be Australian stamps and currency exchange/postage price is different?
You have to put US postage on it for it to return. You can't use stamps for one country in another. Based on the size and weight of a bumper sticker I would say just include an extra $1.50 for Koko to buy an international return stamp. They are $1.15, but that would cover in case it is a little bit over an ounce.