I'm 100% sure about the differences in customs form presence.
Stuff that ships regular-way comes via usually comes via China Post and always with a customs form that is usually somewhat dubious (wrong item description, wrong value, checked "gift" box, etc). This always feels like just I'm buying from an individual seller and importing directly; like the equivalent of how buying internationally from small sellers on US eBay is.
Stuff that ships Standard Choice comes without any of that (and also often gets consolidated into a singular larger bag that has smaller bags from different sellers inside of it). When they're not consolidated, then the individual bags are always re-labeled with a paid USPS mailing label over the original. However it is that my Standard Choice orders are handled, they always show up in my mailbox looking like very-normal domestic mail and there are no customs forms to be seen.
So least in terms of logistics, Standard Choice has every appearance of being very centralized and bulk-oriented -- including within the US.
In terms of certainty: That's all that I'm certain of, and I'm only certain that it is this way for me here where I am.
The rest (eg, how this actually plays out in the future) is speculation on my part. I think my speculation may be correct, but I do not know that it is correct. I feel confident, but I won't be able to know if my confidence is misplaced until after the dust settles. They're obviously free to restructure any or all of this, and are incentivized to do whatever it is that makes them the most money.