I've been careful to keep mine at $22 or under from MBV and I've been OK despite the fact they marked one at over $23 which had been the price before the 10% discount!my last MBV came through with no duty payable yet it was clearly marked as $40 which is over the free limit
I don't bother checking the exchange rate and just try to keep them at $22 or under.
Not a problem with the other US ones I've had.
Think in future I'll get any MBV from Cloudhouse if they have what I want, or concentrates from CV, again if they have what I want