Amazon is the anti-Costco also. We thought about it, and it doesn't really make sense to get a Costco membership when we can lean into Prime more. It doesn't help that we live in a fairly urban area (Ballard in Seattle) and Costco's is pretty suburban.
I'd much rather order some heavy stuff from Amazon to have delivered and walk to the local grocery store for everything else.
A major upside to Costco is you can actually see stuff and you also can walk out of the store same-day. Also I never ever worry about the counterfeit and/or low-quality crap you inevitably get from Amazon. And if Costco sells me something crappy, I drag it back in and don't even have to start up the printer (it's a zombie at this point). Costco has a running rule that they never charge above 10% in profits so I know I'm getting a good deal too.
What about the price-quality aspect? Costco blows Amazon away here IME. Plus there's the fact that someone can become an employee of Costco out of high school and spend their entire career there, with decent wages and benefits. That's not happening at an Amazon Prime fulfillment warehouse.
You can order from Costco on Instacart here in the Bay Area.
This said, there's a lot of quality stuff at Costco (besides their huge wines collection) that you can't find anywhere else.
Generally the bulk things I would have gotten at Costco, which isn't much for our family, so mostly protein drinks, olive oil, and so on. They come in pretty big cases, and it definitely seems like the heaviest thing the Amazon driver is delivering that day.
We still drive to the Chinese grocery for a big bag of rice every once in awhile.
I'd much rather order some heavy stuff from Amazon to have delivered and walk to the local grocery store for everything else.