Real Bad Service

I've handed a Samsung Gear in for repairs in Nov 2015, they've send me an quote which I've accepted and after receiving the invoice I've paid. After several phone calls they promise that the gear would be ready for collection the 15th of Dec, which did not happen. They found another fault and send me a new quote, I've paid the additional invoice, send proof of payment and waited for feedback. On my email to them with the proof of payment I've specifically asked to let me know when I could collect the gear. Didn't get any feedback, phone them again. They rep**** with: We have not received the parts yet. So this is going on and on....today after I insist to speak to a senior, they confirmed that Samsung SA do not have stock and they are not able to tell me when they would have stock. Now my question is: HOW DO YOU QUOTE AND INVOICE FOR REPAIRS WITHOUT KNOWING IF YOU COULD ACTUALLY DO THE REPAIRS!!!