Samsung S6 Edge - Second Motherboard within a year and a half

I bought my phone in February 2016, a few months down the line I had to replace my phone motherboard which Samsung had no problem with because it is still under warranty, However when I had to send in my phone again for the second time (28/08/2017) to repair/replace the motherboard they are telling me "Sorry you don't have warranty anymore" and email me a quotation for R 4275.00. I was told when I purchase the phone that I have a 2 year warranty on my phone. I have requested detailed reports for the repairs and till today I haven't received anything.

I have been a huge Samsung fan for a number of years and this is the first time that I'm having so much of trouble with a phone. I'm refusing to pay for these repairs. The service is **********.