Wrong billing

Last month on the 18th I wrote my review for Rain Internet service and they said to me they have given me discount on my service of 2 weeks service and in real that was the correct amount since I only used the services for 2 weeks. Now today the problem continues because I only used the services for 2 weeks again from the 18th to the 1st which makes it 13 days and I am expected to pay the full amount. I will not pay the full amount because this is wrong I will not accept bullying from the big company like Rain.