Thank you. Actually this all started at as a simple web page [1] where you can plug your invoice into the site and get a PDF of your invoice back with no sign up required. I climbed up the search rankings and eventually was getting tons of requests for more features. That is when I decided to make a premium version. Even today a majority of traffic comes through the Upgrade button on the free tool. The second source of major traffic is referrals from existing customers. To date I have spent $0 on advertising and have had virtually zero coverage on tech blogs. I got lucky that my target customer starts out using free invoicing sites.
What app did you create?
[1] http://invoice-generator.com