The variables in play are certainly many and complex, but I generally subscribe to the theory so-oft espoused by our resident shipping maven, @Roxy:
BSS/MSS is articulated, and manipulated, to satisfy Amazon’s own needs in regards to building brand integrity/recognition - the ‘holy grail’ of “customer delight” - rather than upon any other consideration.
Problems, vagaries, dead-ends, and discrepancies arise because of many factors, as well, but I would opine that the available evidence of many years strongly suggests that many of their root causes are largely attributable to a lack of institutional guidance - i.e., proper leadership AND integration of well-siloed teams in Amazon’s Byzantinely-convoluted infrastructure - which results in poor parameterization of initiatives, and of the automated mechanism(s) deployed to satisfy their goals.
USPS does not have Sunday delivery in all areas. USPS tested Sunday delivery in respond to issues during COVID in areas where package volume increase dramatically during this time. Generally speaking, Sunday deliveries were tested in urban areas. After COVID, several of these areas retained Sunday delivery all of the time and some of them receive Sunday only during the holiday times.
Amazon’s integration with USPS data to give Buy Shipping results has been the subject of many threads for a long long time. Amazon’s results even varies greatly between where you are and where another seller might be. This discussion and comparison is one the @Roxy and us have had for some time. Often what she sees happening and what we see happening at the same time will vary.
All Buy Shipping issues tend to always lead back to Amazon’s coding as these issues don’t seem to happen with other shipping vendors.
To be fair, it should be recognized that SDS - “Sunday Delivery Service” - actually predated the COVID-19 Crisis scare, having been launched the same year (2013) that Amazon threw open the U.S. Marketplace to P.R.C.-based sellers, as a consequence of Seattle’s TPTB’s response to the abject failure of its initial venture to capture market share over there (Amazon.cn).
I just checked 2 items and ground advantage was there. When I went back less than a minute later to purchase the labels, ground advantage was gone. Ground advantage has been switched on and off at least 4 times in the past 6 hours.
Earlier this year, when our friend Sawie predicted the Ides of March (15Mar`24) as a likely expectation for Amazon finally getting it’s act together re: GA implementation for BSS/MSS*, I was initially so-skeptical that I’d have willingly given odds on a proposition to the contrary.
NOW, you couldn’t get me to wager on that same proposition without giving odds.
*
Citational reference to the specified SAS/SellersAskSellers post is available upon request.