While I do not directly recommend this technique, it seems using the PreferenceManager.GetDefaultSharedPreferences is really popular in saving values between different activities. While you can create multiple names that do not conflict between different activities, it can get messy real fast.

Instead of using GetDefaultSharedPreferences, create multiple GetSharedPreferences with different names and thus you can store values with the same names without overwriting your values from a different activity. (Again, I am not a big fan of this technique either for anything more than some simple UI preference settings, see notes at bootom of post)

i.e.

        var title = "stack"; float price = 123.34f; long weight = 23; 
        var editor = GetSharedPreferences ("MyFirstActivity", Android.Content.FileCreationMode.Private);
        var edit = editor.Edit ();
        edit.PutString ("title", title);
        edit.PutFloat ("price", price);
        edit.PutLong ("weight", weight);
        edit.Apply ();

        title = "overflow"; price = 99.99f; weight = 99;
        editor = GetSharedPreferences ("MySecondActivity", Android.Content.FileCreationMode.Private);
        edit = editor.Edit ();
        edit.PutString ("title", title);
        edit.PutFloat ("price", price);
        edit.PutLong ("weight", weight);
        edit.Apply ();

        editor = GetSharedPreferences("MyFirstActivity", Android.Content.FileCreationMode.Private);
        title = editor.GetString("title", "empty");
        price = editor.GetFloat("price", 0);
        weight = editor.GetLong("weight", 0);
        Log.Info("activity1", string.Format("{0}:{1}:{2}", title, price, weight));

        editor = GetSharedPreferences("MySecondActivity", Android.Content.FileCreationMode.Private);
        title = editor.GetString("title", "empty");
        price = editor.GetFloat("price", 0);
        weight = editor.GetLong("weight", 0);
        Log.Info("activity2", string.Format("{0}:{1}:{2}", title, price, weight));

adb logcat output:

[activity1] stack:123.34:23
[activity2] overflow:99.99:99

Notes:

Personally when I see someone saving shopping cart data, using JSON to serialize objects in and out of SharedPerferences string values, etc…, my head starts to pound in a bad way. At that time you should really start looking for an asynchronous, persistent key-value store, like Akavache.