I haven't had to update from Canada in a while (my last Tracfone phone was a CDMA model which does work in Canada (at least it did in Ottawa (*** kind of works, not everything works which is part of the reason I just changed it to a TripleMinutesForLife GSM phone) and any minutes I added on that one would show up immediately as they do when you add minutes when in the states but before that, I'd add my minutes online (from Canada), keep a record of the transaction and the next time I was in the US, I would have to call customer service with the details of the minutes I added and they'd add them then. A bit of a pain especially since the phone wouldn't work immediately when I entered the US but it always worked for me.
Also, FWIW, I have no idea if it's changed but before after your time ended, you had 30 days to add more minutes before you lost your existing minutes (e.g. if you got a 200 min / 90 day card, you didn't have to add more minutes until 119 days later (although obviously from day 91 to 119, you wouldn't get any Tracfone service)). I did this several times when my days would expire during the summer (we don't typically travel to the US over the summer).