Hi, I encountered the exact same situation as you a couple weeks ago. They sent postal mail and it arrived exactly 1 week before my ceremony. At first I was nervous so I called them to get the location. The agent advised me it is fine to go without the letter, the only extra form you have to sign is the media release form. She sent me a list of documents to bring where oath letter is listed as
if available. She told me to print my ECAS status if it makes me feel less stressful
At my ceremony there was someone without the oath letter and after he informed them, they were fine with it.
Just make sure you get the location from the agent and reconfirm the date and time, you should be fine. In the mean time, keep checking your mail box.
Have a great time there like I did!