Amazon wants you to go Prime. It wants this so much that if you still haven’t signed up for Prime's well-known deal—free two-day shipping plus unlimited video streaming for a $99 a year—the company is making it hard to get any kind of a deal on shipping at all.

This morning, Amazon raised its free shipping minimum to $49 for its non-Prime customers for most products. That's up from $35—the first time it increased the minimum order requirement since 2013. At least one category isn’t affected, though: book orders only need to hit $25 to qualify for free shipping.

The company isn’t offering an explanation for raising its order minimum, but one obvious rationale is to push even more customers towards a Prime subscription. Not that the retail giant isn’t doing well on that front: In its last quarterly earnings report, Amazon revealed that paid Prime membership had increased 51 percent worldwide and 47 percent in the US in 2015. At the same time, Amazon’s shipping costs grew 37 percent to $1.8 billion last year—one of its biggest expenses.

Amazon is getting more aggressive with its shipping and delivery initiatives. It's ramping up its one-hour delivery service, Prime Now. And it's reportedly building out a global delivery network that could ultimately put Amazon in complete control of the flow of goods in its supply chain, from factories in China and India to last-mile delivery to customers' homes.

But of all the elements of its logistics strategy, Prime is still Amazon's biggest asset. Now the company is nudging customers even closer to a place where it doesn't make sense to use Amazon without going Prime. For anyone who shops on Amazon even just a few times a year, it’s an obvious deal: fast, free shipping, plus extras like great original shows. And if that carrot doesn’t work, Amazon seems to be saying, maybe the $49 minimum-order stick will.