Invoicing service FreshBooks is part of the software canon for freelancers and entrepreneurs. It's shown up in Lifehacker recommendations from 2009 to 2018, and it's still one of the best ways to stay sane when billing multiple clients. Two years ago, the company rolled out an entirely new version that it had secretly developed under the name Bill Spring. We talked to FreshBooks co-founder and CEO Mike McDerment about building his own company's biggest competitor.