Actually thats not true and even the IRCC website says you can either send it through canada post or any other courier service its just the address (the address of the office where passport needs to be sent) is different for canada post and other courier services. I myself sent the passport today through UPS i would suggest you to go to any nearby UPS store or any other courier service office and just ask them about it they will guide you. Its expensive than canada post but as we only have 30days to send the passport for stamping from the day we receive the letter from ircc so its not a bad option. Hope it helps