They also offer some degree of curation, so your dollar goes further by volume/weight/unit on average, but also in less quantitative ways quite often. I trust what I buy from Costco, but I’ve completely stopped buying from Amazon (many years ago now) because apart from the poor value prop, I simply don’t like the quality or reliability of what’s offered.
This is an important point for me. I am too lazy (too much executive dysfunction) to research products. I trust Costco to do the product research and offer us a good quality/price match up.
Return policy is also pain free. You can pretty much return anything you don't like. Just take it back.
They used to have amazingly generous return policy for electronics. Since some people abused it, bit tighter now.
Amazon largely being a dumb marketplace, a faster-shipping AliExpress/Temu, really makes them easy to drop if you find that shipping speed isn’t super important for those types of products. You can just go straight to the source and cut Amazon out entirely.