I don't think you can use variable defined inside one case block in another case block. Because there is no particular sequence of execution for case blocks. Better approach would be, declare PopupWindow as member variable of the class.

public class MainClass extends Activity implements OnClickListener {

public float goldCount;
Button minionClick;
Button storeClick;
Button storeDismiss;
TextView textGoldCount;
String textTotal;
private SharedPreferences prefs;
PopupWindow pw;

@Override
public void onCreate (Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);

    //Set full-screen
    requestWindowFeature(Window.FEATURE_NO_TITLE);
    getWindow().setFlags(WindowManager.LayoutParams.FLAG_FULLSCREEN, WindowManager.LayoutParams.FLAG_FULLSCREEN);

    setContentView(R.layout.mainlayout);

    prefs = getSharedPreferences("LeagueClicker", Context.MODE_PRIVATE);


    goldCount = prefs.getFloat("goldCount", 0.0f);

    //Linking the variables
    minionClick = (Button) findViewById(R.id.minioncentreid);
    storeClick = (Button) findViewById(R.id.storeimageid);
    storeDismiss = (Button) findViewById(R.id.menudismissid);
    textGoldCount = (TextView) findViewById(R.id.textviewtop);

    //String which will display at the top of the app
    textTotal = goldCount + " Gold";

    //Setting TextView to the String
    textGoldCount.setText(textTotal);
    textGoldCount.setGravity(Gravity.CENTER);
    Typeface tf = Typeface.createFromAsset(getAssets(), "mechanical.ttf");
    textGoldCount.setTypeface(tf);
    textGoldCount.setTextSize(35);

    //Setting onClickListener
    minionClick.setOnClickListener(this);
    storeClick.setOnClickListener(this);
    storeDismiss.setOnClickListener(this);


}


@Override
public void onClick(View v) {
    // TODO Auto-generated method stub
    switch (v.getId()){
    case R.id.minioncentreid:
    goldCount += 1.0;
    prefs.edit().putFloat("goldCount", goldCount).commit();
    textTotal = goldCount + " Gold";
    textGoldCount.setText(textTotal);
    textGoldCount.setGravity(Gravity.CENTER);
    break;

    case R.id.storeimageid:
        LayoutInflater inflater = (LayoutInflater)
           this.getSystemService(Context.LAYOUT_INFLATER_SERVICE);

        if(pw == null) {
        pw = new PopupWindow(
           inflater.inflate(R.layout.storemenu, null, false), 
           300, 
           450, 
           true);
        }
        pw.showAtLocation(v, Gravity.CENTER, 0, 0);
         break;

    case R.id.menudismissid:
        if (pw != null) {
        pw.dismiss();
        }
        break;

    }

Also do not forget to put break statement at the end of first case block :)

Edit 1: You can access view elements of the PopUpWindow by

//Linking the variables
minionClick = (Button) findViewById(R.id.minioncentreid);
storeClick = (Button) findViewById(R.id.storeimageid);
textGoldCount = (TextView) findViewById(R.id.textviewtop);

//Initialize variables from popup window
 LayoutInflater inflater = (LayoutInflater)
       this.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
PopupWindow pw = new PopupWindow(
           inflater.inflate(R.layout.storemenu, null, false), 
storeDismiss = pw.getContentView().findViewById(R.id.menudismissid);

Edit 2:

Official doc for dismiss function says that

public void dismiss ()

Added in API level 1 Dispose of the popup window. This method can be invoked only after showAsDropDown(android.view.View) has been executed. Failing that, calling this method will have no effect.

So your code for case 2 should be

case R.id.storeimageid:
        LayoutInflater inflater = (LayoutInflater)
           this.getSystemService(Context.LAYOUT_INFLATER_SERVICE);

        if(pw == null) {
        pw = new PopupWindow(
           inflater.inflate(R.layout.storemenu, null, false), 
           300, 
           450, 
           true);
        }
        pw.showAsDropDown(v, 0, 0, Gravity.CENTER);
         break;

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You can write a helper class which does it for you. For example:

import android.content.Context;
import android.content.SharedPreferences;

/** Preferences (prefs) manager. */
public class Prefs
{
    public static final String MUSIC_KEY = "music_key";
    public static final String VOLUME_KEY = "volume_key";
    public static final String USER_ID_KEY = "user_id_key";
    public static final String FIRST_HELP_KEY = "first_help_key";
    public static final String SECOND_HELP_KEY = "second_help_key";
    public static final String CHALLENGE_HELP_KEY = "challenge_help_key";
    public static final String PREMIUM_KEY = "premium_key";
    public static final String GAMES_PLAYED_WHEN_STATS_SENT_KEY = "stats_games_played_key";

    private static final String FILENAME = "preferences";
    private static final SharedPreferences prefs = Const.context.getSharedPreferences(FILENAME, Context.MODE_PRIVATE);

    public static boolean getBoolean(String key, boolean defValue)
    {
        return prefs.getBoolean(key, defValue);
    }

    public static float getFloat(String key, float defValue)
    {
        return prefs.getFloat(key, defValue);
    }

    public static int getInt(String key, int defValue)
    {
        return prefs.getInt(key, defValue);
    }

    public static long getLong(String key, long defValue)
    {
        return prefs.getLong(key, defValue);
    }

    public static String getString(String key, String defValue)
    {
        return prefs.getString(key, defValue);
    }

    public static void putObject(String key, Object value)
    {
        final SharedPreferences.Editor editor = prefs.edit();
        if (value instanceof Boolean)
        {
            editor.putBoolean(key, (Boolean) value);
        }
        else if (value instanceof Float)
        {
            editor.putFloat(key, (Float) value);
        }
        else if (value instanceof Integer)
        {
            editor.putInt(key, (Integer) value);
        }
        else if (value instanceof Long)
        {
            editor.putLong(key, (Long) value);
        }
        else if (value instanceof String)
        {
            editor.putString(key, (String) value);
        }
        else
        {
            throw new IllegalArgumentException(value + " can't be inserted into SharedPreferences");
        }
        editor.commit();
    }

}

Later, when you want to read an integer value, you can just do:

final int lastSentGamesCounter = Prefs.getInt(Prefs.GAMES_PLAYED_WHEN_STATS_SENT_KEY, 0);

Writing:

Prefs.putObject(Prefs.GAMES_PLAYED_WHEN_STATS_SENT_KEY, 10);

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as suggested by Squonk, I used SharedPreferences and I could solve my problem with this:

MainActivity

    SharedPreferences settings = 
        PreferenceManager.getDefaultSharedPreferences(getBaseContext());
    SharedPreferences.Editor editor = settings.edit();
    editor.putFloat("Brightness", adjusted);
    editor.commit();

All others activities

@Override
protected void onCreate(Bundle savedInstanceState) {
    //...
    SharedPreferences preferences = 
        PreferenceManager.getDefaultSharedPreferences(getBaseContext());          
    preferences.registerOnSharedPreferenceChangeListener(prefListener);
}

SharedPreferences.OnSharedPreferenceChangeListener prefListener = 
    new SharedPreferences.OnSharedPreferenceChangeListener() {
        public void onSharedPreferenceChanged(SharedPreferences prefs,String key) {
            if (key.equals("Brightness")) {
                WindowManager.LayoutParams lp = getWindow().getAttributes();
                lp.screenBrightness = prefs.getFloat(key, 0.8f);
                getWindow().setAttributes(lp);
            } 
        }
    };

Thanks for help me Squonk.

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Not sure on the deck of cards, but store the money as a float. It is not an int. displaying it is simple as since java can make a float a string by simply adding the float to an empty string

str = "" + flt; complicated? How?

protected boolean store_cash(){
   SharedPreferences settings = getSharedPreferences("GENERAL", 0);
        SharedPreferences.Editor editor = settings.edit();
        editor.putFloat("cash", mCash);
        return editor.commit();
}
protected void load_cash(){
 SharedPreferences settings = getSharedPreferences("GENERAL", 0);
 mCash = settings.getFloat("cash", (float) 0.0);
}

with that it's just a matter of setting the title of the textview

protected void display_cash(){
    TextView cash = (TextView ) findViewById(R.id.cash);
    cash.setText( "$" + mCash);  //formatting may be required..  google is your friend
}

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