Assuming it's the US we're talking about, the federal minimum wage is $7.25, which means that if every worker involved is paid at the minimum wage, you incur a cent of labour costs every 4.97 person-seconds. AFAICT, most Amazon workers are paid substantially higher than the federal minimum wage. And that's just labour costs.

While Amazon is efficient, "fractions of a cent" is probably the wrong order of magnitude for even the most efficient order.

That's not even mentioning their additional overheads, like the cost of fuel for their idling van as they drop off your potato.