Android POS Add-on For existing website

We are in need of a POS solution for our existing website. The website uses the Yii Framework. Its a food portal with multiple merchants selling their food. We want a solution to build a pos system where a merchant can use the same menu updated on the main site to create an order for a client. The app should be as an add-on for the existing website, where we will be able to connect the sites we build for our clients. All in all, you will be using the same database as of the website Same login panel as of the website You should connect the app via json api calls, all in all, it should be control from the main website so you dont have to create any admin panel for the pos system to update menu or add items as the whole menu will be updated on the website Should be able to use the same payment system we already have and with same credentials so no new payment setup either all included in the site some mockup and more information included in attachments