No exchange for guaranteed tyre

I bought 4 tyres at mintys tyres in jhb and was told that any issues with the tyres I can exchange it at any of the branches. After relocating to Durban (2 months) I realised one of the tyres had a bubble. I went with my invoice to the mintys tyre branch on Umgeni road and spoke to the manager which said that it was due to negligence as the bubble was on the side which is not the case. This is pathetic as I've spent alot of money on not 1 but 4 Dunlop tyres by them, they even had the audacity to encourage me to pay for a new one. So disgusted in the service and attitude of the people aswell