I've never had Prime. Instant gratification isn't a draw for me and I can build up a shopping cart over months between orders. Funnily enough, about 75% of my boxes come with Prime tape on them. It used to signify expedited handling. Now it's just a branding exercise.
There's an entire Amazon fulfillment business that many don't know about - if you move enough merchandise with them you get the ability to use their shipping network, which can be pretty darn cheap.
So often it's the company/seller using Amazon for fulfillment; but sometimes it's just someone doing arbitrage between Amazon and eBay.
