Hi,
if you are using a purolator to send your passport which is a good thing because they have a good service and you won't have to worry about your passport been lost,simple just include a prepaid canada post express post envelope inside the purolator envelope along with your passport and whatever documents you will be mailing.
no the porulator guys doesn't need to know,why would i....make sure before you include the return envelope to write down the tracking number so you can use it online to check the return envelope movement when its been sent out.
you can use any courier to send your passport to ottawa such as purolator but the return envelope MUST be canada post xpress post envelope or else your passport will be returned through regular mail and you won't be able to track it plus you may face long delivery time or even lose it.
you will get a detailed instructions email along with the passport request,please wait for it before making any steps.