Game App: ******* business practice?

Trying to fully pay up my game account in full using their own Game app: The App will not let you pay the full amount so I paid 1c short of the full amount. (it actually tells you that you are unable to pay the full amount)

Then the following month I get charged fees for having an outstanding balance! But, it's their own system that doesn't allow you to pay up your account in full.

Surely I should not be liable to pay the new bigger balance??? (R94)

Surely this must be *******? I will pursue this. This is disgusting business practice.