Absolutely. I think the unpredictability is one of the areas that is particularly pernicious to small businesses that are trying to deal with the border, because they don't know exactly when it's going to be enforced.
You're precisely right. We understand that the effective de minimis moves with volume, and actually moves depending on where the volume is coming through. For something coming through Vancouver, the effective threshold might be much higher, because the volume of goods coming in from Asia is much higher there than the volume coming through Toronto.
As regards the Christmas season, if you want to be less likely to be assessed for duties and taxes, buy something at Christmas. The volume goes up, and the ability of the CBSA to stay within budget and assess packages means that their effective threshold goes up.