Google should introduce a new option for App types, I suggest they call it "in App Payment". Currently the option is Free or Purchase when a developer lists an application in Google Play store. The suggested option can be used for kinds of Apps that are free to install but you may / have to purchase something after installation via the application.
It is not fair to those Apps that are completely free nor to the customers. Costumers have to read the full-length description to find out if a specific App is really free or just the installation is free. For an example, Hushed App, is listed as a free app but you have to at least buy a number within the app to use it. One of the most famous App of google play, Whatsapp, has similar story. Using this App, you have to buy annual registration after your first year. However, there is another App called Viber which is totally free. You don't need to pay anything in the future using the App. Imagine how many people might have considered using Viber over Whatsapp if Whatsapp has been registered as a "in App Payment" option and not a free one.