You can also call them or visit one of their offices in your area.
Good suggestion, I just happend to see this being dealt with at customs terminal outside airport, possible it can be done elsewhere, don't know.
For background, we got the BSF form stamped on arrival; then we had to go to airport customs when our shipment arrived for the forms to be re-stamped (still not clear why this extra step, but it is what it is).
At customs on this second trip of ours, there was a couple that had not done the step on their own arrival; they claimed there hadn't been customs involved at all when they arrived [reasons don't matter]. Same story, basically - they were still told it was their own problem/fault, they should have found a customs officer and had the forms processed, blah blah blah - much time wasted discussing this point.
Eventually the customs officer decided the ritual humiliation/insistence that there was no fault attributable to CBSA was sufficient and started work on helping them fix it. (Note, at this point he was quite a stickler about all other paperwork and evidence - whereas when we did it on arrival, there were basically no questions; although possible the circumstances were quite different too).
In other words, it's discretionary on the part of the officer. Don't waste time arguing the point, you missed it, you didn't know, you were tired, you were confused, it's your fault, your mover didn't tell you when you needed to show this, is there any way at all to fix this? Obviously no guarantee this will work but clearly it is possible.
And do it soon. I'm quite sure that if thye have any thought this is stuff being shipped after the PR arrives in Canada, they won't do this.
