Solved
As I have to print and post my return, I want to wait until I get the notification from CRA before paying. I don't know whether me ticking the "5 days" box means that I now have to pay them within 5 days.
If you've indicated when you file that you intend to pay your tax balance due within 5 days, they'll hold the processing of your Notice of Assessment (NOA) until they get your payment so that when they do send out your NOA, it will reflect that you've paid.
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