I was always under the impression that the opposite is true, as long as you’re not paying extra for them to deliver on Shabbos or Yom Tov it’s okay. But that assumption was based on the Psak at a place I worked at way back. They shipped orders via UPS (and maybe Fedex) — and UPS Ground from NY to NY generally doesn’t take more than a day. Saturday, and Next Day deliveries cost extra, so they wouldn’t ship that way on an Erev, but regular, not expedited shipping was allowed anytime.
Nowadays with Amazon I hate Saturday deliveries as I have to peek out the window to see who’s buzzing and run down to manually open the door (as early as 8 am) so I try to avoid it. And I’ve had Postal delivery guys lie (this was during the week) and note they unsuccessfully attempted delivery when they never did so they don’t miss Amazon’s “by 8:00 PM” promise.
My point is that their compliance is far from 100%, so I wonder if that could be a mitigating factor. OTOH IIRC you get a $10. reward if they fail to deliver as promised and you complain, so there’s definitely pressure to deliver on time for all involved.