This app uses activities for purchasing list, grocery list, expenses list and favorite items. It is but practical that a user have list organized in a phone.
The purchasing list can be used like a purchased order where orders are recorded with item or description, quantity, unit price and total amount.
The expenses list records item, unit price and total amount spent.
A frequent user can opt to use this app like a calculator where you can save data and use is it for reference. A good example of this is when you are ordering in a restaurant. You record your order one by one. When the bill arrives, you can check that each food is what you'd previously ordered, with the total bill amount of course.
In expenses list, an example is when you are working with a travel allowance. You record each trip with gasoline, parking, meals, toll fee, personal essentials, etc. You may then recall if you still have an extra budget.
The favorites list is used by the user to record frequently bought items. In this case, you can compare prices from one store to another.