It's been a while since I didn't comment on anything on HN, but since you are doing something that is really hard and I have some experience with accounting and banking, may I offer you some tips that you might want to investigate further:

- Regulation: Some countries (and some US States) do have guidelines on how this type of software must be constructed, make sure you are compliant to those regulations if it's the case. Example: IIRC you need permits to even start building a POS solutions in some states.

- Integration: An usable accounting software must include HR or have integration with common HR software used by most companies. HR is one of the most complex parts of accounting. Most clients will have outdated solutions they are comfortable working with and won't change completely to your software, make sure you can integrate with tools they want to continue using. Only fight the old if the option you are offering means task elimination. Don't forget you need to integrate with banks, but that's easy.

- Customization: Human systems are not pretty. There are things that do not make sense to accounting and are often loaded with emotional decisions, human relationships and corporate strategy. Sometimes a software just streamlines the human mess, make sure your software can be customized by your clients to a certain point.