Why does it matter if it's the same number as a line item on the invoice vs you pay it to UPS upon receipt? It's the same price.
So now I gotta be home 100% to receive the package, instead of it getting it dropped off at a neighbors if I happen to be out/at work/...? And I hope they picked a shipping company that takes card/allows online payments and not someone who expects me to pay in exact cash.
And that's the happy path where everything worked correctly, and not one where something in the paperwork is wrong or got messed up and now you get to try figure out what exact proof the shipping company wants to release your package/believe that they got the amount wrong/... Or it gets shipped in a way where it ends up stuck with actual customs and you get to deal with them. If a seller has their shit in order it's relatively unlikely to happen, but well, random small-ish companies not necessarily do, and if it goes wrong its really annoying.
+ of course if the seller has an actual legal presence here it makes it clear consumer rights as I expect them apply.
I'm not someone who absolutely won't order from abroad if I want something, but a local source or a known distributor just avoids a whole category of potential issues. And computers are the kind of high-value item where people really want to avoid them (and some of the simplifications aren't available, e.g. I think the most straight-forward pre-payment mechanism for sellers is restricted to low-value parcels).
We do not have any of these considerations in the US, which is perhaps part of why so much stuff comes directly from abroad.
But $800 (I think that was the value?) de-minimus indeed makes a lot of cases easy.