But if you order from Amazon on Friday with Amazon guaranteeing overnight delivery (or guaranteeing two-day delivery for a Thursday order), you know in advance that they are guaranteeing delivery on Shabbos.
If you order on Thursday for Saturday, they usually technically could deliver it before Shabbos, so their delivering it on Shabbos is “ada’ta d’nafsgei” so not a problem.
The same idea could be true in the winter even if ordering on Friday. They could start the process before Shabbos,band deliver it after Shabbos, and get the package to you on Motzaei Shabbos without having done any melacha on Shabbos. The fact that they might actually do melacha on Shabbos isn’t your concern in that case as well.