With GSP what happens is your seller ships to the plant in Kentucky-- by USPS or whatever.
There her responsibility ends.
GSP does not ship to Canada by USPS. GSP is NOT part of the mailstream.
Once in Canada, GSP has been transferring mostly* to Canada Post.
BUT
GSP also is using tracked shipping.
And apparently tracked parcels are being moved first-- before LetterMail and untracked parcels (mostly from Asia).
I'd still allow three weeks from your seller to your doorstep though.
*We'v seen reports of CanPar and FedEx delivering GSP shipments in the past. The reports were more like complaints, but that's normal. We don't hear about transactions that go smoothly.