It's using the language from the Bezos shareholder letter [0]. I recommend reading it, because I think the vision for how to run a company is very good, even if Amazon isn't living up to it.
Returns and shipping aren't "Day 2" features necessarily -- they can be part of a "Day 1" company.
Rather, my complaint is that Amazon has created programs like Fulfilled-by-Amazon (FBA) and Featured-Merchant-Algorithm (FMA) that are poorly implemented and anti-consumer:
1. Co-mingled inventories are a fundamentally anti-consumer feature, because they violate consumer control and mean that bad actors can impact the experience of buying from good actors, which undermines consumer trust. The only purpose it serves is simplifying the logistical problem for Amazon. The reason Amazon thinks this is a good idea is because they have converted to data-as-proxy thinking, where they no longer holistically understand consumer needs, they only optimize proxy values like delivery speed. The whole feature reeks of abandonment of Amazon's leadership principles [1] in order to optimize performance metrics. That's "Day 2" thinking in a nutshell.
2. The FMA is similarly problematic because of the fact it essentially hides a vital decision about purchasing from the consumer, and forces buying on Amazon to feel adversarial -- I have to doublecheck that Amazon isn't sending my business to a fraudster and they make it hard for me to control that, except for a small box I have to check every time. Here, Amazon failed to advance the market with new and innovative consumer tools that would allow customers to help police the market (eg, by crowdsourcing an optional fraudster blacklist). That kind of "Well, the data tells the whole story, let's just optimize our metrics" with opaque algorithms is a very "Day 2" way of doing business -- a failure to innovate is stagnation.
I could go on about these for a while, but the entire design of the Amazon marketplace is a stagnant mess -- it's no longer innovative, and their reliance on optimization to metrics has allowed people to game them, turning it into a quagmire of people trying to cheat the system (counterfeit, bought reviews, etc).
Amazon is devolving into a mess of their own making, by operating in a "Day 2" paradigm, rather than innovating solutions to hard problems.
[0] https://www.sec.gov/Archives/edgar/data/1018724/000119312517...
[1] https://www.amazon.jobs/principles