Finally received a reply from Canada Post:

Hi,

The sell online application was upgraded this past weekend to only accept HTTP(s) requests, all TCP requests will be rejected. There was a fix released yesterday morning as the application was rejecting HTTP requests as well but has now been resolved. That being said, in order for to access the application you will need to make changes so your application is sending HTTP(s) requests only. Let me know if you have any questions.

Thank you,
Angad Chawla
Project Implementation Officer | Canada Post
Pretty huge change to make without informing anyone (no mention of it on the SellOnline main page). I'll do some fiddling this weekend to see if I can get the original version working again (SellOnline has integrated box support for much more accurate quotes, Zucando's doesn't unfortunately). Will post again if I have any success -- any help is definitely welcome!