Delivery to the U.S.A. and internationally
Instead of being able to view the actual signature image online, only the name of the individual who signed for the item will be available for viewing online.
The signatory name option is included at no extra charge for Priority Worldwide items and Xpresspost-International prepaid envelopes, and is available for a fee for Xpresspost–International items sent to specific destinations. -- from canadapost.ca
http://www.canadapost.ca/web/business/mailing-and-shipping.page#USA
Apparently the signature option is automatic. You don't have to do anything else.