Despite not really needing very many bulk family sized items, my wife and I like shopping at Costco. You get hands on the items and can inspect them, and generally the quality of what Costco sells is well selected and very high (even if it's not fashionable). e.g. grab a random bottle of wine out of their wine section and you can guarantee it'll be pretty drinkable, even the $6 bottles. Their customer service and no questions return policy has more than paid for the very reasonable membership cost ($55 a year, which you'll probably just save on toilet paper purchases alone). But selection is slim and often hit and miss as favorite items rotate in and out of the stores.

Compare to Amazon, which sells multiple versions of the same SKUs from slightly different retailers (with slightly different shipping methods), and then hundreds of slightly different items all with a wide variance in quality and you spend hours reading reviews and comparison shopping. It's "ok" to do from home, but I frankly have better things to do with my time most of the time. However, for those really hard to find items, or ones that Costco simply doesn't carry, it's totally worth to shop at Amazon.