To anyone who may be thinking of doing this as of late 2021:
IRCC backlog + Afghanistan has thrown things into even more chaos, and CBSA doesn't seem totally sure how to navigate it. My recent experience:
Applied online, after 6 weeks, flew to the border to try there. CBSA looked over my stuff, and started my permit. When he accessed my UCI, he saw my online application, and told me it was approved, asked for my POE letter. Told him my account says 'processing' and had no POE. I do know there'd been a hang up on my employer's side that may have stalled the finalization, but either way CBSA called IRCC who told him that they couldn't unmatch the offer from my online application, and that I either had to wait until I got the POE or withdraw and apply for a new job offer number, and then flagpole. Let me in as a visitor, confirming that my notes said APPROVED and suggesting to call IRCC and that it shouldn't be too long...
Two weeks later, still a visitor unable to work, and neither IRCC nor CBSA can tell me whether or not they can issue the permit, or how to get out of the limbo this has put me in. Getting a hold of my visa office was an amazing feat, I must admit but ultimately they still gave me no help. According to them, my application is 'in queue to be reviewed'. So, I'm leaning towards CBSA making a bad call, but really I don't know.
So, proceed at your own risk or whatever.