The "law" is that stuff that's illegal in Canada can't be bought in the USA. So no switchblades, gravity knives, or bali-songs or variations.
Further it appears that the CBSA is trying to socially engineer any flipper blade out of existence. There is a perceived very high seizure rate for that kind of knife despite the fact you can buy them in Canadian Tire etc. without issues.
For tax and duty the keys to having it clear with no extra charges are for the value to stay low, under a couple hundred USD and for it to ship via USPS. If you get something $500 via FedEx or UPS you'll get dinged the full tax and duty charges.