Costco shipping is "free," but the prices on the website are more than in the store. I'm pretty sure it's based on weight; heavy stuff has a more pronounced markup. But, it saves you a trip, and the prices are low to start so it still may be significant savings. There's also a web-only deals section for clearing out inventory (I picked up some Philips Hue lights for half off).

I don't purchase from costco.com regularly, but the selection is generally the same as in-store and a bit more. The shipping is free, but slower than Amazon and faster than a couple weeks.