I understand how receiving that notice so soon after payment can be surprising. It's possible the notice was sent to you before Fido received your payment. If that is the case, you could disregard that notice. Your best option would be to check your account again. If there is no outstanding balance shown then Fido would have received your payment.
You should note that this is the community forum and not intended as a venue for customer services. If you wish to discuss the matter you would need to contact customer service. In addition, they can also be contacted via Live Chat, Facebook, or Twitter. Those methods can be accessed via the contact page posted above.