Agree with millercarbon’s points. Hell, sometimes with the savings you can buy a SECOND, if you get ripped off on the 1st one or some reason, and STILL come out ahead. And I really think getting ripped off is pretty rare in this game (always shop smart). For those of us who can buy mail order and install it ourselves, the reality is that there is very little value to dealer / distributor support on a cartridge. I build a long-term relationship with my local dealer for tube amps, speakers, turntable, etc - and it works out great! Both parties happy. With cartridges the value just isn’t there. You buy the cartridge, and if it’s going to be defective it’s most likely going to be immediately apparent - so you request a refund for DOA, which applies no matter the source (PayPal or CC dispute as a last resort for sellers who won’t play ball). After that, it’s probably either going to die by your own (error) hand, or through use over a long lifetime (hopefully!).
The only exception to this is if you miss out on any sweet trade-in or rebuild deals that may be provided only through the "authorized" channels (it would have to be a hell of a sweet deal though) - or manufacturers that won't service retip / rebuild cartridges without a paper trail.