MailPieceFactory.cs

public class MailPieceFactory
{
    private readonly Dictionary<char, MailPiece> _mailPieces = new Dictionary<char,MailPiece>();

    public MailPiece GetPiece(char key)
    {
        if (!_mailPieces.ContainsKey(key))
            _mailPieces[key] = BuildPiece(key);

        return _mailPieces[key];            
    }

    private static MailPiece BuildPiece(char key)
    {
        switch (key)
        {
            case 'P': return new Postcard();
            default: return new Letter();
        }
    }
}