Money making tactics

I am trying to purchase a part for my ALVA gas heater and they have confirmed that they have the part available; however, they refuse to sell me the part. They are insisting the i purchase a service voucher for R485.00. I have tried to explain it to them that i can replace the part, as it is just two *****s and a nut. They still refuse and insist that i purchase the service voucher, which is a clear money making tactic.