Amazon’s subscription program, Prime, has emerged as a key pillar of Amazon’s e-commerce and streaming business. For $99 per year, Amazon Prime members can access streamed movies, TV shows, and Amazon’s original productions in addition to free two-day shipping and one-hour delivery on certain orders. According to a recent Consumer Intelligence Research Partners report Prime has 54 million U.S. members, up 35% from 2014. And 38% of American households are members, according to research from investment bank Cowen.

Sprint, which is the fourth largest carrier in the U.S., is the first wireless carrier to start selling Prime as an add-on.

Sprint customers can opt to add Prime to their monthly plan for an additional $10.99 per month. That ends up being roughly $32 more than purchasing Prime through Amazon. However, Prime members who purchase the subscription from Amazon don’t yet have the option of purchasing Prime on a monthly basis.

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A month to month option could be a more attractive option for users who want to stream a particular show or movie, or if they anticipate a large amount of purchases on Amazon’s marketplace in a given month.

It’s still unclear the exact number of Prime members in the U.S. Amazon said in the third week of December that it had added 3 million new members without stating publicly the number of total Prime members.