I am searching for a template to create an Invoicing app for android or ubunutu platform for a small business. Please share with me any thoughts about creating this app. Do not want to signup on any web based app platform. Appreciate any input on this idea.
App Inventor is a web based app platform, where you can build apps that you can run locally on your devices 
If you don't want to sign up you can use MIT App Inventor but you have to remember your login code else you can not access your apps anymore.
You must first learn bookkeeping.
Learn bookkeeping as in the business accounting. I already know some it, just trying to create some app to use on a mobile device to create invoice for customers we deliver to. This invoice needs to have total amount and we have to get signatures as well. Do you know if any template I could bring into the design platform.
That leaves you with just your phone.
What happens if you lose it?