+1(514) 937-9445 or Toll-free (Canada & US) +1 (888) 947-9445
Just an update:

My wife mailed her application via Xpresspost (no signature required) to the P.O. Box. Canadapost on-line tracking confirmed it was received and she recently received AOR via e-mail.

So you can definitely use Xpresspost for the P.O. Box address
 
Hi,

I sent my citizenship application by Xpresspost to the courier address ((Sydney. NS) with signature required. However, on Canada post website, I received "A signature has been captured for this item, however, the signatory has requested that the signature not be displayed on this website. Is this normal?
 
Hi,

I sent my citizenship application by Xpresspost to the courier address ((Sydney. NS) with signature required. However, on Canada post website, I received "A signature has been captured for this item, however, the signatory has requested that the signature not be displayed on this website. Is this normal?
Normal. I had the same with just initials. MK. I knew its Mike Krswda since he has previously signed off many of my past packages related to PR applications etc.