Hi,
I am a Python developer and have used Flask to build several web-applications; I have no prior experience with A2billing API, but I had a quick skim at its documentations and I believe I can help get your issue solved.
You haven't given any sufficient details in the project description, however, so I hope we can discuss the problem first on chat before I can give any promises! Would you kindly send me a message on chat so that we can further discuss your project ? Thanks in advance.