Thanks for your reply. It seems that the response anyone gets is based on the mood of the agent at the IRCC Call Centre. The lady we spoke to today was not helpful. She was very short with us and did not want to assist, despite us being as friendly and cooperative as possible. She never offered to check with the web team or anyone else.
Both times when I used the web form, I confirmed that the file sizes were under the limit. There seemed to be no issues. So I'm at a loss as to what is going on. I mailed a hard copy to Mississauga a couple of hours ago. I only hope it actually arrives safely and that there are no more delays.