When I mailed my application for the certificate I was told by the PO that the US Dept. of Homeland Security was sometimes holding up mail leaving the US for a foreign country, including Canada. He told me that the PO was having a hard time answering questions from people wondering where their mail had gone and that the USPO saw this as an unnecessary bureaucratic policy. He said that if my packet had been a bit larger he was certain they would hold it up. This may account for the delay in acknowledging receipt of my application. Just passing the info on.
With the reliability (or lack thereof) of both the USPS and Canada Post, I would try to minimize delays. I mailed my package trackable in Scarborough ON, which is not that far from Sydney NS. If I had mailed it from the US, I would have mailed it Priority Mail International or Priority Mail Express International with tracking. Both give it a leg up on most other mail classes (especially first class mail) on clearance priority.
If you apply for a passport from the US, or in Canada for delivery to the US, they charge $100 CAD extra. When they send it though, they send it via a Canada Post/Fedex combined service (FedEx from Canada to the US) where delivery is guaranteed the next morning before 10:30AM. Not even the Passport Office portion of CIC/IRCC trusts regular mail...
The actual Citizenship Certificate (CIT 0001E) arrives via regular standard Canada Post/USPS mail, although I was not held up on receiving it (postage was $2.75 CAD paid by CIC/IRCC on a large envelope).