Continuing from part 1, part 2, and part 3, let's now talk about using in-app purchases. The technical details and some useful design thoughts can be found on How to support in-app purchases on the Windows developer center. What I want to add to that discussion are some observations from a few other apps.
First, with the obvious case, you can use in-app purchases to turn certain features on. This includes turning off ads, meaning that you use an in-app purchase for this purpose rather than converting a trial to a paid app as in part 3. Here, you're thinking about how a user can convert money (their money) into added capabilities.
Second is thinking about how to introduce another level between the purchases and capabilities. This is a strategy used in a number of games that have their own in-app "currency" (coins, points, etc.), with which you can buy extra capabilities. Often you can earn by playing the game for a long time, but if you're impatient you can convert real money into in-app currency and thus shortcut the process. In other words, you're letting the user choose between patience/perseverance and a little bit of their money.
Two of the most popular games, both by HalfBrick Studios, are great examples in this regard: Fruit Ninja and Jetpack Joyride. Fruit Ninja offers the Dojo within the app where you can acquire different blades, backgrounds, and special (and temporary!) powers (like a bomb deflector). Some of these you can acquire by performing special feats within the game, like finishing with a specific number of points, slicing specific fruits in a certain sequence, and so forth. You can also acquire them buy exchaning "starfruit points" that are gradually earned by playing the game. But then you can also make an in-app purchase of extra starfruit points, allowing you to then purchase the add-ons with those points.
So if you're not as patient as my six-year-old son has been to save up 9000 points for the King Dragon Blade, you can just spend a couple of dollars and get it right away. Jetpack Joyride does something similar. By playing the game and completing missions you earn coins with which you can buy all kinds of extra gadgets, powers, fun clothing, and so forth. Again, you can earn all the points you'll ever need just by playing the game. But you can also just flat-out purchase coins with real money.
Because the Windows Store does not yet provide for in-app consumables (an in-app purchase you can buy multiple times [update: this is available in Windows 8.1]), Jetpack Joyride offers a number of one-time purchase options: 20,000 coins for $1.49, 50,000 coins for $2.49, 100,000 coins for $3.49, 250,000 coins for $5.99, and 1,000,000 coins for $13.99 (which would allow you to buy everything). So again, if you're willing to spend, you don't have to be patient. Jetpack Joyride also offers a brilliant in-app purchase that I thought was well worth $1.49: the "counterfeit machine" which automatically doubles all the coins you earn when playing the game. This is totally worth it!
So for in-app purchases, I strongly encourage you to look at what other apps are doing, especially the most popular ones, as they offer good insights into designing around this monetization approach.