If you want me to send you a final grade, or current grades on exams, homeworks etc, use one of the following two methods. Likewise if you want to discuss grades or other sensitive matters.
Email me your request. Sign or encrypt your email using PGP. I'll reply in encrypted format.
Type up a description of your request. Don't assume that I might be able to guess what your postcard is supposed to contain! Include an envelope that will return it all to you with my response.