Bad service and worthless guarantee

Sandton Appliances repaired our Defy oven. They wanted paying upfront prior to making the repairs. Within 3 weeks the rubber seal they had installed had snapped because it had been stretched too tightly by the technician who installed it and the oven globe they used burnt out after the second use. When reported to the manager Vuyo, they hide behind fine print on the back of the invoice stating they don't guarantee certain parts even though on the front of the invoice they have a 90 day guarantee. Seals don't break after a few weeks unless not properly installed or poor quality parts are used. Seems they don't guarantee poor workmanship either. My request to speak to the owner Dan also seems to have fallen on deaf ears.