Dancu said:Thanks a lot for your information on my question!
Re-reading their instructions for mailing it seems they really prohibit any option other than just regular mail (for Canadian return addresses). So I might just do that since otherwise they may delay the whole process of passport return.
What I understood from the PPR email text you posted, I think the do not want to go to any courier service (DHL, fedex etc.) for mailing the passports. If the xpresspost pre-paid envelope is from US post and the regular mailing is also through US post, then it does not matter if they use xpresspost or regular mail. They will give all mails to US Post for delivery. If it is an xpresspost then they save of postage stamps as well.