Over charge

On my latest invoice I had an additional R170,00 charge , when this was queried I was told this is because of the billing that went through two days later on the 17th, this is the first time ever a charge was done in this nature , I asked if a credit will be done in the next period and was told that it won't be. Which I can't understand ?? I have been a customer with Supercall Cellular for more than 10 years and a average bill of R1800,00 per month, this is completely unacceptable and a ******* , can't wait for my contract to finish so I can go to a new service provider