Customized billing application

I am looking for a developer to design a customized billing application that is written in JavaScript,php, is mobile compatible, and has admin user accounts. The application must be built from the ground-up with comprehensive capability for customers and administrators to access and manage their needs. It must support all major mobile platforms, and be tailored to fit the specific needs of the project. The development process should include UI/UX design and testing to ensure it is reliable and easy-to-use. The end result should be an application that effectively streamlines billing processes and covers all the necessary aspects of customer and admin user accounts.