Very dissapointed

Good day hellopeter and fellow readers . it all started with a Samsung that I took out on contract the 27th October 2016 however the phone was heating up so I had to send it back. 2 weeks later I got a new phone and had to get a new invoice however the guy said he will mail it to me but he never did so I have been battling for more then a month and I dont know what to do. I was at Samsung waterfront on the 26th October 2017 to book my phone in for a screen replacement as the guarantee is only valid for a year. Sadly I had the first invoice and Samsung couldn't accept the phone. The same day I Contacted mtn for the second invoice however they did forward it to me but I couldn't possibly have a hard copy. I asked the receptionist if I could email it to someone and she gave me an email address to forward it so then they accepted my phone. After weeks of struggling and waiting they haven't repaired my phone. According to unitronics they have received my invoice too late :( but I specifically went in a day earlier in case I needed something. I have been in contact with zeldean useless . I have waited for a response from her after the numerous emails we have shared and today 11/12/2017 when I contacted Samsung call centre they said she has resigned. Also the lady said that damages has been done to the mother board now I have to wait longer and also have to pay. This was suppose to take only 21 days and now I'm stuck up until today with a phone I had and still pay my monthly fee + needs to pay for extra repairs when all I wanted was a screen replacements can you guys help me please