To encrypt your files on your Android device you can choose "Upload" in the BoxCryptor app after you tapped the option button or you browse to your file using a file explorer. Then tap and hold on your file and choose "Send" and then "BoxCryptor". After that your file is encrypted on your device and sent to your Dropbox account.
Of course it is also possible to download BoxCryptor on your laptop and use your existing BoxCryptor folder or create a new one. The limitation of BoxCryptor for Windows is a 2GB encrypted folder limit. So if you need more space you have to purchase the Unlimited version.
All your apps which you have bought in the Android Market are stored in your Google account. So if you uninstall an app you can install the app later without the need to pay again for it.