The App Store can sometimes get a little buggy and prevent you from making an in-app purchase.
When this happens, you need to reset it's cache by following the steps below:
1. Logout of the Mac App Store (Store -> Logout).
2. Quit the Mac App Store (Press red X icon in top left corner, alternatively press CMD and Q keys together).
3. Quit Memory Clean (right click on menubar icon and select 'Quit').
4. Then open Terminal and run the below command (you may be prompted for your password).
OS 10.10+ and above Users run: sudo pkill -f CommerceKit
OS X 10.9 and below Users run: killall -9 storeagent
5. Re-open Memory Clean from your Applications folder and try to purchase again. It should work.
If you have any further issues, please contact us via the support form.
Please Note: If you are trying to purchase outside of the USA Mac App Store, there can sometimes be a bug with Apple's servers where you will be told you need to change country. Simply press OK and then sign in again with your iTunes Account. Then quit Memory Clean and reopen it and make the purchase attempt again. This time, it should process it correctly.