Amazon’s Book Box for kids is available to all US Prime members

The age-based selections include two hardbacks or four board books.

In May, Amazon launched its Prime Book Box for kids as an invite-only program and now the company has opened it up to all US Prime members. With the subscription service, customers can get a set of books selected for their children based on their age and they can choose to have it delivered every one, two or three months. The box, which costs $23, will include either two hardback books or, for ages two and younger, four board books.

The subscription is geared towards children up to 12 years old. Parents can preview their selection ahead of time and select alternate titles from a list of curated options or they can let Amazon choose for them. Subscribers can also skip a box at any time. Amazon says the box saves customers up to 35 percent off of the list price, but it there's ever an instance where the selected books are available on Amazon for less than $23, subscribers will be charged the lower price.