A cross platform Mobile app - Tips Calculator type application.

I would like to have created a cross-platform mobile app for Android and Apple (and hopefully Windows Phone but not necessarily) All phone sizes and tablet as well. It is a Tips type app that will be a Calculator, Splitter, and also support PayPal. PayPal experience required. Similar products are primarily these ; 1 [login to view URL] 2 [login to view URL] 3 [login to view URL] but also these as other examples of functionality ; i - [login to view URL] ii - [login to view URL] iii - [login to view URL] iv - [login to view URL] PLATFORM I was thinking of React Native or similar. Something that is easy to maintain and runs well across platforms. The UI can be specific to the Android and iPhone, the Engine should be generic. Xamarin is a possibility but I am aware there are issues with slowness and compatibility as I have read. I am open to discussing options. I will want both the Android and iPhone versions delivered. SOAP will be required also to communicate with the server. VISUALLY This must be a professional looking app and I will ask for design examples/suggestions. This is a real app not just simple static pages so you must have done this sort of thing previously and also please show me a couple of examples. I will be more specific when we discuss. Time Frame is important as is experience ans capability. I will ask that you commit to a time frame and stick to that because other matters will rely on this time frame. BACKUPS Daily or at least every two days I will require a full backup of all code so I can compile at this end.