Playing Customs Games with Our Northern Neighbors

Lately, i have been confronted with buyers from Canada who have insisted that i declare an artificially low value for an item that i am selling so that they can avoid the heft customs duties in Canada. In one case, this request was made AFTER the buyer had already paid me. In a subsequent case, i told the prospective buyer up front that i would not mis-state/under-insure the the item and this put an end to our discussions (presumably, the customs fees would have been so significant that we couldn't resolve this by simply "pricing it into the deal.")

I have no problem selling to buyers from Canada, but i do have a problem providing false statements on a Customs Declaration. Am I over-reacting?