There is no exception in the Federal Switchblade Act for using Fedex, UPS, etc to ship switchblades to civilians or any individual persons. I used to be under the same misconception, but Kniferights.org set me straight. And since they are directly involved in getting switchblade laws repealed in various states (with several successes), I trust that their lawyers know what they're talking about.
The misconception is that the Federal Switchblade Act prohibiting the shipping of switchblades across state lines applies only to the US Post Office, with "common carriers" mentioned under "exceptions", but that doesn't mean those "common carriers" (Fedex, UPS, etc) are legally permitted to ship switchblades across state lines to any Tom, Dick, or Harry, instead, the "exception" in the Act is intended to provide "common carriers" with protection from prosecution if and when they unknowingly transport switchblades across state lines, and to exempt them from prosecution when they ship switchblades across states lines in relation to an official government contract.
The bottom line is- it is illegal under any circumstances to transport switchblades across state lines for the purposes of "commerce" (buying/selling) UNLESS the person/company doing the selling/transporting is acting in regards to an official government procurement contract.
Also, contrary to popular belief, there are no exceptions for active duty military, members of law enforcement, or people with disabilities. The military can purchase switchblades, and so can law enforcement departments/agencies, but again, the vendors can only sell and ship them LEGALLY to a military branch/unit or law enforcement department/agency with an official government procurement contract.
As far as the forms that Knifecenter and other vendors sometimes require buyers to sign, they are not official government or legal documents, and they have no legal standing or authority whatsoever. They are just an attempt by the vendors to cover-their-ass in the event that they get caught selling/shipping switchblades without an official government contract, so they can say "See, we did everything we could to avoid selling switchblades to the wrong people". But the reality is, those forms won't offer the vendors any protection, because the laws says- no selling or shipping switchblades across state lines without an official government contract. And a vendor would certainly know if they were in possession of an OFFICIAL government contract.
I have never attempted to purchase a switchblade from Knifecenter, I've heard that they are strict about who they sell to. But if they had one I wanted to buy, I'd give it a try. The worst that they could do is deny the purchase. It's not like they're going to call the FBI on you.
I have successfully purchased several switchblades from other online vendors (I live in California too), and those vendors shipped the knives across state lines. To my knowledge, it is the person doing the sending that is violating the law, not the person doing the ordering/receiving. I've seen cases of vendors being prosecuted by the Feds for violating the Act, but I've never heard of a buyer getting prosecuted, and I looked.
DISCLAIMER: I am not a lawyer. When in doubt about the law, consult an attorney who is highly knowledgeable about knife laws. I would also recommend visiting the Kniferights.org website and read their explanation of the Federal Switchblade Act, which is what I put my faith in. I am only human, and their is always the possibility I could be wrong about something.