Bad service ever

Nissan Kempton park, I had purchased a V-arm for a Datsun Go and was not told about the return policy or what so ever, found that it was wrong, never opened and not a mechanical part. Called them and was requested to bring it back with a letter from the bank and invoice slip. I have been calling for the past 3 to 4 weeks the manager Mr Lee keeps on promising to call me back every time but I always have to call back with no direct response. Worse customer service experience I have ever experienced. Please assist