We bought 2 single mattresses from Healtcare and Mobility for R28 350

We bought 2 single mattresses from Healtcare and Mobility for R28 350.00 (twenty eight thousand three hundred and fifty rand). It is now not 3.5 years later and the one mattress is so soft and saggs in in the middle so badly that I now have permanent lower back pain. I removed the covers to wash and was amazed to see that it was two difference mattresses - one gray and one pink? It is the grey one that is now useless. I contacted the providers and was told the guarantee of 5 years was only on the workmanship and that the grey one was for 80 - 140 kg (I weigh 85) and that was why it was different but were never asked if we wanted a 'softer' mattress. I was offered a brand new firmer mattress for R6500