Humble Bundle, the promotional site that lets consumers pay what they wish for bundles of DRM-free content, generated $6.1 million in revenue on 31 e-book bundles released in 2015, its second year offering e-books. About half, 16 bundles, were comic-book-related and brought in just over $3 million. Humble Bundle has raised about $272 million in total revenue from digital games (and now e-books) since it was founded in 2010.

For each bundle sold, Humble Bundle divides its revenue among the publisher, the authors, and itself (a 15% “tip”), and it also allows consumers to designate part of their payments for charity. In 2015, about $1.2 million of the e-book revenue that it generated was donated. Since its launch, Humble Bundle has given more than $70 million to charities. Among the groups that it supports are the Comic Book Legal Defense Fund, Doctors Without Borders, Make a Wish Foundation, and Camden’s Concert.

Charles Brownstein, executive director of the CBLDF, said financial support from HB has had a “transformative” impact on his organization, which monitors issues related to the freedom to read and First Amendment rights. Thanks to HB, he said, “CBLDF has been able to dramatically expand our program. Their funding has enabled us to hire new staff members, which has increased our ability to do our program work.”

Originally launched as a promotional vehicle for digital games, the Humble Bundle site attracts millions of visitors each week looking for the discounted bundles. HB works with publishers to offer a bundle of titles promoted to consumers for two-week periods for whatever they wish to pay. In 2014, the first year the site experimented with e-books, it generated $4.75 million on 18 e-book bundles.

Kelley Allen, Humble Bundle’s director of e-books, said the site averaged $200,000 per each two-week e-book promotion in 2015. A recently completed promotion, the Pathfinder RPG Book Bundle, finished with 77,503 bundles sold, generating more than $1.2 million, HB’s bestselling bundle to date.

Publishers are happy to work with HB, she said. The site is a “highly visible discovery place for their titles and authors,” and she said publishers earn “fairly respectable revenue for a brief two-week promotion.” And though she does not have data on cannibalization, she said anecdotal information suggests “digital sales tend to stimulate print sales and vice versa.”

Allen said some of the most popular promotions were comics, backlist titles, audiobooks, and small-press bundles. Last year’s top-selling Humble Bundle promotion was a collection of out-of-print prose and comics works from the 1980s and 1990s by bestselling author and comics writer Neil Gaiman.

The Neil Gaiman Rarities bundle sold 32,340 bundles, raising $634,774.73. Other big-selling book bundles included the Image Comics Bundle (29,780 sold; $457,385), a Doctor Who Audiobook Bundle (22,031 sold; $288,942), and a Brainiac Book Bundle from No Starch Press (34,366 sold; $458,517).

Allen said HB plans to do more dual book bundles (two promotions each week rather than one), and she expects to launch 52 book bundles in 2016. And look for more original content: “at least three launches next quarter will be completely new hot-off-the-press releases from bestselling authors,” she said.

Over the next year, Allen said upcoming releases include HB’s first manga bundle, an LGBTQ bundle for Pride Month (in June), and the debut of several new comics publishers, including Aspen Comics and a yet-to-be-named “biggie.”