During the Black Friday to Cyber Monday weekend two years ago, about 8,000 people for the first time used Sezzle Inc.’s payment system when they shopped online.

A few weeks later, Sezzle executives mentioned that number to some bankers visiting their office in downtown Minneapolis. One of the bankers let out a giggle.

“It’s kind of rude to do that,” Charlie Youakim, co-founder and chief executive, said as he remembered the moment recently. “But I was thinking to myself, ‘I don’t think that person understands compound growth.’ It’s not as laughable where we’re at now.”

Last month, hundreds of thousands of people paid for $113 million in goods from retailers in the U.S. and Canada using Sezzle. Well over 2 million people have used Sezzle’s at least once this year, the company’s data shows.

A shopper who chooses Sezzle instead of a at checkout pays one-fourth