Normally yes. The app is designed to work with an API.
You may still use the app and benefit of ftp upload. Some functionality might be limited for instance sales screen or payment request.
Depending on your custom prefferences for specific features, it should take somewhere between 2 weeks and 3 months or more.
You can cancel anytime the maintenance of the app without extra fees.
No, we provide designs at no costs. Of course we need to have a clear picture of what you actually need, therefore some sketches might help.
At a very low cost you may receive the source code for the app. It will include all the features you benefit already.