GooglePlayReceiptFix.cs

// The Google Play receipt JSON comes with a payload, which contains a string of nested JSON structure.
// In order to deserialize that structure, apply this method to the raw JSON string BEFORE you
// deserialize it to an object.

public static string FixJSONFormat (string json)
{
  return json.Replace (@"\\\", "").Replace (@"\""", @"""").Replace (@"""{", @"{").Replace (@"}""", @"}");
}