In the olden days…AC could easily be had at a discount. from both resellers like LA Computer Company, and secondary markets like eBay. One could simply make an online purchase, receive the code via email without any accompanying packaging (quick and easy!), then contact Apple, provide the code, and have the plan recorded.
But then a fraud problem emerged -- people had figured out how to generate false AC codes, so Apple started to demand either a physical proof of purchase, like the box, and/or a a purchase receipt from an Apple reseller. Not just an eBay transaction receipt.
The code-only AC plans being sold became very risky, with buyers not knowing whether they were fraudulent, or whether Apple would even accept them., so the market shifted to "boxed" plans, which stood a greater chance of acceptance. This was at least a decade ago and I guess persisted for some time.
(Another issue stemming from Apple's laxity, which nobody else could be blamed for, was that it would accept legitimate, but never redeemed codes from previous models, and apply them to more current models which called for more costly plans.)
I believe it was the shift to AC+ that resulted in an even bigger change -- no more standalone AC purchases, and no more old boxed plans, with AC only being sold by resellers accompanying a hardware purchase, on the same receipt, or from Apple directly within the allowable grace period if it was not purchased at that time.
Even though those little AC cards may still hang in some stores, they are worthless without being "activated" (like a gift card) by an accompanying hardware purchase, and cannot be purchased on their own.
I put that to the test not long ago with an older AC-only card, in trying to skip the AC+ accident coverage, and the card could not be rung up by itself without a hardware buy.
So now, the only real options for AC is to purchase it with the machine, or from Apple itself afterward.
Superficially at least,
Best Buy still appears to offer it, but the inspection and proof of purchase requirement, as well as no assurance they'd accept proof of sale from another reseller make it not worth the hassle or the risk of wasting time if they refuse to sell AC to a machine not originally sold by them.
In short, Apple's clampdown of AC sales has completely upended how things used to be.