So I thought @amazon used to have UI to tell them that an order marked as delivered ("left at mailbox") wasn't actually delivered. Now I can't find any such UI and the help content basically says "contact USPS". Is the assumption now that I just pay for the thing again?

I mean, it's not a big deal for me to just buy more soap since the first container of soap went missing, but it's also not really how I thought Amazon's e-commerce model worked.

I think the button only appears after a few days. Because often the parcel was delivered to a neighbor and just forcing you to wait a couple of days can help the package magically reappear without intervention from their side.