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Rest assured that you did nothing wrong on your end. The amount displayed on your invoice will reflect the account balance owed at the time, the bill is populated. That said, any changes made mid-cycle after the invoice is issued, can impact that balance.
For example, if a credit was applied on our end after you received your bill. In that case, the amount to be paid before the due date should be your updated account balance, shown on the main page of your Fido.ca profile (Overview tab).
Since you now have a credit on your balance, this means that you've paid more than the amount actually owed. This credit can simply be left there and it will be applied to lower your next invoice.
Hope this clarifies, and let us know if you have any questions.