Here's an outline of an Android app that includes user registration, login, product browsing, and invoice generation functionalities:

1. User Registration:

   • Create a registration screen with fields like username, password, user type.
   • Implement validation checks for the registration form.
   • Store user registration details in a database.

2. User Login:

   • Design a login screen with fields for username and password.
   • Verify the user's credentials against the stored data in the database.
   • Allow access to the app upon successful login.

3. Product Listing:

   • Create a screen to display a list of available products.
   • Fetch product data from a backend server or local storage.
   • Implement search and filtering options for easy product discovery.

4. Product Details:

   • Show detailed information about a selected product, including name, description, price, etc.
   • Include an option to add the product to the shopping cart.

5. Shopping Cart:

   • Design a cart screen to display the selected products.
   • Allow users to adjust quantities, remove items, or proceed to checkout.

6. Checkout and Invoice Generation:

   • Implement a checkout process where users provide shipping and payment information.
   • Calculate the total cost and generate an invoice for the user.
   • Store the invoice details in the database for future reference.

7. Admin Panel:

   • Create an admin interface for managing products and user orders.
   • Allow the admin to add new products, update existing ones, and view user invoices.
   • Implement authentication and access control to secure the admin panel.

Remember, this is just a high-level overview, and you would need to implement the actual code and logic to achieve these functionalities. Additionally, you'll need to set up the backend server, database, and API endpoints to handle data storage and retrieval.