You're paying for convenience. Or I am at least. Knowing you're going to get the item in ~2 days makes the difference and without needing to pay for the cost difference anyway (+ some usually) when you get to the shipping page.
Yet fast shipping doesn't inherently cost much extra. The human effort involved to pack the box, load the van, and ring your doorbell is exactly the same if they do it tomorrow or next week.
And most delivery companies drive around every neighborhood every day, so they can't exactly say "we save money by only delivering on Mondays, Wednesday's and Fridays".
From a naive perspective it might. It doesn’t work like that in practice though, as it wouldn’t be cost effective since it’d require lots of extra overhead. Most any complex system will face a trade off between latency vs throughout. Shipping isn’t much different mathematically from a system of queues much like a database or network. There’s plenty of math for it: http://www.cs.binghamton.edu/~dima/ep01.pdf
Optimizing costs requires keeping work queues as full as possible which requires increasing latency.
