Melted phone refuse to repair under warrenty

I send my tablet (Plum) in for repairs after it was only a couple of moths old so its still under warranty, So what happened was I put the tablet on charge with its original charger then I smelt something it started to melt I took it in the pep cell where I have bought it they send it for repairs. months after that I went to see were is my tablet so they send it back saying I need to pay R 500.00. So I was confused as why do I need to pay if it is still under warranty so it got send back. So today I get another call saying I need to pay as the warranty is voided due to physical damage (It melted) so now it is my responsibility to pay before they will fix my phone I emailed them and their response was as follows (I copy and pasted the email) Good Day, Chantelle

I hereby acknowledge the receipt of your email.

Please be advised the fact that the device is melted repair center regard it as physical damage, as per MTN Repair center terms and conditions any physical damage voids the warranty. So now i want to know what is the warranty then for it was not my fault it melted and i could not have prevented it from melting.