Istore Sandton bullies

Based on an Apple chat online with a bot, I was advised to replace my iPhone X 256gb at 84% health. I looked an online appointment only to be told when I arrived, that they in-fact have no batteries in stock. Why allow the appointment then? I was informed that batteries were available about a week later. I took my phone in and asked them to check my telephone speaker as the volume was a little low. Other than that the phone worked perfectly. The phone was booked in and a quote for around R7800 followed which I reluctantly accepted. On receiving the phone back I started having issues with multiple apps and it was very slow to respond to any functions, and it wouldn't turn off by holing volume up and side button. I had to hard restart. I took it back in and was told it was software related and they had restored to factory. I reinstalled all my apps and the problem persisted. I did a sim swap to eliminate any unforeseen issues. I took the phone back in and the diagnostics showed clear. The faults however were evident to the Istore agent so the phone was again booked in. I have now receive ed a quote for R13600 to replace the unit as it is beyond repair. I had a phone with a a faint speaker which cost me almost R8000, and after paying, now I have a unrepairable phone. I offered to pay in the difference between what I had paid for the repair and what latest quote was, but they will not entertain it. This means I have wasted R7800 to get my phone from serviceable to unserviceable.