Changed sim rental of a swiping machine from R99 to R198

I received my merchant statement and realised that money payable to Nedbank is too high for the R6000 swiped for month of December. When i scrutinise I see that they changed sim rental of a swiping machine from R99 to R198. They never asked me if i accept these new terms. Then they start contacting me after hours to resolve my problem only to tell me that it is after hours so they will call me the following morning. This woman who called me now say that the charge for that rental is R198, she is not showing me where I agreed to it instead she say"'DID THEY NOT TELL YOU"" I should get told like I am Nedbank subject or slave?