Wont return my laptop to me

My laptop stopped booting. It automatically shut down while i was using it, for no reason, it was charged. I took it to a technition, who confirmed a problem but said he couldnt diagnose it without opening it which would void the warranty. I sent the laptop to Acer, as i could only find one approved service provider who is so far away that i thought is would be quicker to send it to acer in joburg. Acer collected it from me through a third party courier and after having it for two weeks claim it is in working order and that i have to pay R800 to get it back even tho it is still under warranty