When u are sending by courier through FedEx/ UPS/DHL to their office directly then it should be sent to below address as I mentioned earlier:
IMMIGRATION SECTION
HIGH COMMISSION OF CANADA
One George Street #11-01,
SINGAPORE 049145
But when u are sending to their
mail box by mail system then the address should be as per canadachocaccon :
Adresse d'expédition : P.O. Box 845, Robinson Road, Singapore 901645
Please check the link of SGVO:
http://www.canadainternational.gc.ca/singapore-singapour/visas/update-how_mettre-comment.aspx
i guess this may clear ur confusions now...