Help Me L8ter's payment structure

So, if you want to make money off an application, you need to actually have some sort of payment structure.. Help Me L8ter is no exception. Most apps in the iOS and Play stores follow one of three ways

1: Paid App from store.

 

Probably the easiest approach, you purchase the app off the store for a one-off payment of some small amount of money.
Pros:

• Quick guaranteed purchase
• Code is easy to implement

Cons:

• Users may not download due to initial payment. No Trial.
• One time payment

2: Advertising in app

The user downloads the app for free and a small ad is displayed somewhere in
the application.
Pros:

• Zero cost to user
• More downloads
• Ongoing payment to developer

Cons:

• Clutter in application 

3: In app purchases

The user downloads the application and gets a small trial period. The user can then purchase the application if wanted.
Pros:

• Easy entry.
• Ongoing payment to developer

Cons:

• Additional complexity to user (purchase screen)
• Additional code

After considering these options, I was leaning towards either Advertising or in-app purchases. I ended up choosing the in-app purchases.
The idea is that you, the user downloads the app. You get 50 "Starts" - that is you can start and stop the timer 50 discreet times. After that has expired, there's a store where you can purchase more starts. I have not fixed the pricing yet but I am imagining an "if you buy more, you pay less" scenario. I also am planning an unlimited option for users.

The implementation on the Setup screen. The Start count is shown on the button.

The shop screen is a fragment attached to the main setup page. You can drag down the page, press the shop button or you will automatically navigate there if you run out of starts

The shop screen, pressing a button will bring up the in app purchase screen

The video might give you a better understanding also.