We have been using this system in one of our projects for about five years. It is convenient and easy to understand, with a client for sellers, the ability to connect to a cash register, and offline functionality. Although stability sometimes leaves much to be desired, overall everything is fine. There is a free plan, and if your project doesn't generate much income, even the basic plan will be affordable for you. The only downside is that the iOS app currently works poorly; it's better to use Android devices or just the web version.
