There were a few speculations on here, so I thought I would provide some answers.
Questions about barcodes: IRCC does specify where they want them included. Will they return the application if they’re somewhere else within the application? Highly unlikely. I’ve yet to see a case myself on this forum where that happened. As long as they’re there in some sort of order you should be fine.
Addresses: IRCC states very specifically what they want for addresses. If you abbreviate the province will they return it? Short answer, yes they can. It’s happened before. Long answer, most of the time that likely won’t happen. There is always a possibility that you’ll get the person who’s a stickler for protocol and didn’t have his morning coffee. It happens, but again not a common reason for return.
Cover Letters: Not required. There’s nothing stopping you from writing one, but as a general rule anything you would have said in a cover letter should already have been stated somewhere within the application, particularly with straightforward cases a cover letter would simply be redundant.
The most common reasons I’ve seen for returned applications are: missing documents, Schedule A not being filled out correctly, missing signatures, or something from the checklist including the list itself is missing. (Remember you need to write N/A next to anything not applicable) If you have not missed these things, relax and don’t stress. Doing so isn’t going to help you in the long run.
For those still getting ready to send:
Double, triple, and quadruple check your applications before sending them in and do so on different days as you’re more likely to catch things you missed if you step away from it for a bit. Having someone else you trust look at it helps, too.