Is it better ettiquite for a man's name or woman's name to go first?

Let’s say I’m mailing something to Mary and Joe Smith. I know that the envelope should say Mr. and Mrs. Joe Smith.

a) What if Mary kept her maiden name? Who goes first?
Mr. Joe Smith
Mrs. Mary Jones
123 Main Street

or Mrs. Mary Jones
Mr. Joe Smith?

b) If I’m writing to them casually, do I say Dear Mary and Joe, or the other way?

c) And finally, do Joe and Mary Smith request your presence or do Mary and Joe Smith request your presence ?

Thanks for your help!