I got a new phone while my old plan was still ongoing so I had to pay for the remaining...

I got a new phone while my old plan was still ongoing so I had to pay for the remaining balance and the plan charges for the remaining bill. I paid everything the employee at Fido West Edmonton Mall location told me to pay and she even calculated it herself. She gave me a fixed amount to pay and assured me I don’t have to pay anything else until the month of August. Yesterday ,July * **** I received a bill saying I have to pay $**.** because I didn’t pay enough the day I got my new phone. I called *** and got in touch with a rep and he kept on telling me that I did not pay enough that is why I still have a remaining balance of $**.**. Why am I paying extra when I was told I’m already paying for everything and have nothing to worry about until August? I should not be held accountable for your employee’s mathematical error and therefore should not be paying that amount.