When the USPS receives a package, it does not count it as a delivery day.
So if you sent out something Monday, they consider 2-3 from Tuesday. So your package would arrive Thursday or Friday.
So techically it's 3-4, not 2-3 as they state. However, if the package fails to arrive 4 days later from pick-up. (IE shipped out on Monday, but doesn't arrive until Saturday), then the shipper is entitled to get a refund back. As a receiver, I've been able to get some refunds back from a shipper because the package came late.