I will be paying my fees soon as well and was wondering about this.
According to the CIC's website, the
Process Flow for paying the fees online show as the last step: "Once your payment has been approved, an official receipt is displayed. You must
print two copies of this receipt and fill out by hand the Payer Information on both copies. Please attach one copy to your completed application and keep the other one for your record." (emphasis mine)
So it seems that the only thing you need to do is print two copies of the receipt as displayed on the screen, fill out the Payer info on both copies and send one copy in with your application. The "official" emailed receipt is probably another proof of payment to keep for your records.
Regards.
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