i'm not a mister

Got an invitation in the mail today addressed to "Mr. (my last name) Cassie" (if you don't know me, my last name is a common boy's name). Clearly the person who sent the invitation is not someone I know (it's an invitation for a party in honor of someone I know)., but still. I don't understand why people constantly need to rearrange my name. When everyone else's name is written first name first, why do people assume that my name was written last name first? Sheesh! And aAfter almost 25 years of this, I still have no clue how to politely say "Yo, moron, my first name is Cassie. They did not write my name backwards and everyone else's forwards, you just chose to ignore what was written. I AM NOT A BOY". I need to get married to someone whose last name is clearly a last name... In the meantime, how do I reply to this one?