A website, such as jet.com is out of "sight out of mind". Unless it becomes a habit (like Amazon has for many people including me).
Costco has several advantages over a jet.com. And the Devil is in the details in terms of what made Costco a success.
1) It's not out of site in any way. You can't miss Costco if there is one in your neighborhood. You can easily miss jet.com or not think about it unless it becomes a habit.
2) Costco succeeds also by doing what Walmart does in part. Once you are in the store they get you to buy things that you didn't think that you needed. And more of it then you thought you needed. This was the concept behind Makro Self Service Wholesale in the 80's (the place that Tom Stemberg of Staples interviewed at and got the idea for Staples for. [1] (Ntim, I was at that store when it opened..)
3) Employee culture. Can be done (Zappos) online but I would say more of a benefit in a physical store (my weakest point I will agree). [2]
[1] http://www.forbes.com/sites/danschawbel/2012/07/05/tom-stemb...
[2] http://www.businessweek.com/articles/2013-06-06/costco-ceo-c...