The only worst case is that customs would require commercial invoice. It is a simple form (google it) and just contains what is inside and what is declared $ value. This problem happens to US residents too as they need to send the passport to Canada from US.
http://www.canadavisa.com/canada-immigration-discussion-board/threads/passport-held-by-fedex-commercial-invoice-required.499972/
Although it is reverse, your case could be treated similar as well. Because FedEx does put a commercial invoice behind the label when we send the passport from US to Canada.
But the problem here is yours is regular mail. I am not sure what is the courier company name that does this. Rather than trying to contact CBSA, you should try to contact the courier company (what ever it is) to understand what is going on. They might be reaching out only to sender and not you. CIC will not submit this form, and you will have to do that on behalf of CIC. You somehow need to intercept the tracking and figure out this.