This is why I only sell items worth posting in pre-paid satchels and I also lodge them over the counter and get a receipt. If you post them in a red box you have no proof of postage. The receipt protects you, the buyer and makes Australia Post responsible (which is who really should be!). I find it also saves charging for signature on delivery, which if you charge for it puts buyers off with higher postage costs (I do add this for items of extra value for insurance coverage as well).