Poor communication for car rentals

Avis advertised a 3 months car rental special and I went on the 20th of August I was told to pay an amount of about R3300 for group A vehicles, I went again on the 19 of September for the second month and I payed the same amount and now I went on the 20th of October for the last month of the special I was told I have to pay an amount of R6300 because things have changed 2 weeks ago and I was never informed or prepared.

Talk about convinience!!