Amazon hides the Buy Now button on items eligible for same-day or next-day local delivery, showing "Add to tomorrow's delivery" instead. This puts it back.
The buy box form is still on the page. Buy Now is that form posted to
/checkout/entry/buynow with two extra fields (submit.buy-now,
isBuyNow=1), so the button adds them and submits it there. You land on the normal checkout page for that item
alone; nothing is ordered until you click "Place your order".
Chrome: chrome://extensions, turn on Developer mode, Load unpacked, pick this
folder.
Tampermonkey or Violentmonkey: buy-now.user.js is also a userscript.