A simplified point-of-sale system in C# linked with my web applications codeigniter PHP

I need system for identifying weaknesses in the relevant C# programming language for my web applications codeigniter PHP ignition code. The sales shortcut system includes essential screens needed by the sales man and sales supervisor. It features a version of their office system that allows it to operate offline or with internet access via daily, weekly, or monthly scheduled code, as selected by the system administrator. A trial period of only seven days. Activation of the merchant account is done through the license number. The check is done through the database. Key system specifications: 1- The application must be compatible with the GS web application codeigniter php through synchronization with the same database, In Google Play, two applications are added, the first application is GS-POS and the second application GS-Restaurants is the same, with some advantages between them. 2-The application should work without needing the internet connection With a limit of the duration of the application to be used without sync and without the internet. 3-The application should support point of sale system Supports screen size 15.6 inch or other + screen with an 8 digital customer price display (customer price display - Multiple printers and quick printing of the invoice when pressing the payment button - opening the cash drawer at the same time as printing the invoice) NFC card reader and Selling through network connectivity, With the difference between the two applications in the GS-Restaurants need to add a map of the restaurant tables, specify the ordering table, print copies of the invoice for the kitchen and the rest of the restaurant, different design of the printed invoice, and some other additions. 4- The application is controlled through a web application system.(Verify the accuracy of the login data by checking the account status/customer license/subscription expiry date/subscription type/How long the app allows you to use without syncing ) 5- All data is matched between the web application and the c# application through synchronization.(Add/Edit/Delete/Print Reports/Shortcut Settings....etc). 6-Sending system notifications and billing via app notifications and via WhatsApp / email... Taking into account the minor differences between the GS-POS & GS-RESTAURANT applications and everything else the system needs that was not mentioned in this request. Note: The application will be on the seller's device, so it must contain everything the seller needs from the web application system, and application management will be through the web application. Provide the complete source code files.