The online retail giant has started a promotion that offers customers the opportunity to get up to $50 off its line of Kindle, Kindle Paperwhite, Kindle Voyage, and Kindle for Kids Bundles. Those who opt for a standard Kindle e-reader, for instance, can now buy it for $50, a $30 savings on its regular $80 price tag. The Kindle Voyage, which is regularly priced at $200, is getting a $50 discount, bringing its price down to $150.

However, there’s a catch. In order to qualify for the reduced pricing, customers must be members of Amazon Prime, the company’s $99-per-year service that offers free two-day shipping and access to its streaming music and video content, among other perks. Non-Prime customers will still need to pay full price.

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Amazon’s (AMZN) Kindle e-readers are among the bestselling hardware products in the company’s massive online marketplace. The Kindle line offers users the opportunity to ditch print books and buy or rent e-books. Those books can be stored on the Kindle device of choice. Amazon’s Kindle devices differ by their features and screen sizes, and their prices reflect those differences.

Amazon’s new offer doesn’t apply to the company’s highest-end Kindle, the $290 Oasis. It’s unknown why Amazon left out the Oasis from the offer and the company did not immediately respond to a Fortune request for comment on that.

On its site, Amazon said that the new Kindle offer is “good while supplies last” and noted that shipping charges and taxes may still apply to a purchase. As of this writing, all of its discounted Kindle devices are still in stock.

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