Mango - No reply

I booked 2 flights return to Cape Town from Jhb. I also booked a car with Avis from the Mango website. Once the booking was made I received an email confirming my booking. On the confirmation the car only had 200km for the rental period and I needed to upgrade this. The Mango website has the option to change any of your booking but it would not accept my changes I needed to make. Therefore I contacted the Mango office 3 times by email and 2 times by phone. I am still awaiting any response from them. There has not been any reply email not even an automated one! The lady on the phone told me to make the changes with Avis, Avis told me to make the changes with Mango as that is where I made the booking!

To me this is not acceptable. If you ignore your client, how would you expect them to come back for another flight?