On an afternoon this spring, James Hirschfeld, a founder of Paperless Post, was at the company’s Lower Manhattan office surveying moodboards for digital invitation designs. They included materials for forthcoming motifs like New Victorian, a collection inspired by 19th-century décor, and a line by Annie Atkins, a graphic designer known for her collaborations with the director Wes Anderson.

As Mr. Hirschfeld examined the collagelike boards, he recalled a meeting about the design of new children’s invitations. “Someone said, ‘Dinosaurs are out, owls are in,’” he said. “And I thought, Is this my life?”

For the past 15 years, it has been.

Mr. Hirschfeld, 38, with his older sister, Alexa Hirschfeld, 40, started Paperless Post in 2009, when they were 23 and 25. He was a senior at Harvard and she was working at CBS as a second assistant to the anchor Katie Couric.

Since then the company has sent some 650 million invitations, according to its own metrics, has grown to employ a full-time staff of 110 people and, as of last year, has been immortalized in a “Saturday Night Live” sketch. Paperless Post has also earned fans in the heritage stationery businesses it sought to disrupt, collaborating with brands like Crane and Cheree Berry on digital products.

Its approach of combining the flourish of physical invitations with the ease of digital correspondence has been adopted by several younger companies, among them Electragram, a digital stationery business developed by the editor Graydon Carter and his wife, Anna Carter; HiNote, a similar business started by Alexis Traina, the wife of a former United States ambassador to Austria; and Partiful, a platform with a faster-and-looser sensibility that has resonated with members of Gen Z.