The tech giant now counts 90 million U.S.-based Amazon Prime subscribers, researcher Consumer Intelligence Research Partners (CIRP) estimates. The company’s study, which includes data from 500 Amazon customers, estimates that Amazon Prime subscribers spend $1,300 per year, nearly doubling the $700 per year the average non-member spends on the e-commerce site.

According to CIRP, 63% of Amazon’s (AMZN) customers are Prime members, and in the past 12 months, the total subscriber base has grown by 38%.

Amazon Prime is a $99-per-year service that gives the company’s customers free two-day shipping and discounted rates on overnight delivery. Prime customers also get access to Amazon’s streaming video library, free e-books, and more.

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For Amazon, the value is clear. At 90 million subscribers each paying $99 per year, the company generates nearly $9 billion in revenue before those customers even buy a single product. And since they spend an average of $600 more per year than their non-member counterparts, they also appear to be loyal customers that help Amazon’s bottom line.

Best of all for Amazon, there’s no indication that many of its members will leave anytime soon. In fact, CIRP found that 95% of current Prime subscribers say they’ll either “definitely” or “probably” renew. That’s up from 94% of Prime members during the same period last year.

CIRP conducted its study between July and September. All of its subjects were Amazon customers who made a purchase during that period.