How is it possible

I recieved a card on the 04/10/17 but was dated by stamp 15/09/17 , to collect a parcel from the Eden Park Post office . I went to the post office on saterday the 07/10/17 , the guy lucky that assisted me said he could not find the parcel . The explaination he gave me i still dont understand , the card says the parcel will be sent back within 30 days , 30 days has not yet passed. They say that people who expecting parcels come to the post office without a notification and they give them the parcel ... how can u do that , i waited months for a parcel i purchased on ebay , now they cant even give me a proper explaination .. What kind of service is this !!! Who is going to refund me for my purchase !!!