[sokoloff]

I'm sure that there's an aspect of what you say at work, but I can also imagine that they have a certain amount of "pick and pack" capacity at their fulfillment centers.

By choosing 2-day, you're forcing Amazon to pick and pack that order today probably, tomorrow at the very latest.

By choosing "no rush", you allow them flexibility to fulfill from a wide variety of centers, on a wide variety of days. Maybe there's a weekly cycle in order volume and no rush orders may allow them to even that out with a more constant labor force.