Simple POS for small vendors -- 2

Want to develop phase 1 of android app or Mobile website which does following. 1) Fetched Items from the database and displays large 2 or 3 rows of button on the single screen of a phablet/tablet. (Screen shot is attached) 2) Clicking on the button increases the quantity for the item by 1 (alternativly we can open a small hover over to input qty or have a + button to increase quantity.) Idea is that it should not take more then few seconds to do this operation. 3) On the side it should display a panel which displays all the items ordered with quantity and price and total at the end. The same panel should display big Pay button. 4) Clicking on Pay button will show possible payment options. (Cash, Credit and some others) 5) Cash and credit will have no further processing. It will just be stored as credit card or cash payments. 6) I will share other payment modes which will require some API integration and will require generation of QR Code for customers to make a payment. We will start with at least two such payment methods in phase 1. 7) All these transactions should be stored in database to generate various daily/weekly/monthly reports 8) Need login with access levels. (Currently only one access level. but for next phase we would need admin who can see all the reports, manage inventory, add/edit/delete items, payment methods) Note: All the server side interaction should be REST based apis and input/output should be clearly documented Speed is of the essence here. So entire operation from order to payment should be very quick and ideally should finish within a minute. You can look at the sample screen here ([login to view URL])