Upside down was a way of saying "head over heels".
One set of definitions:
Right side up = business
Upside down = lovesick, lonesome or I love you
Sideways, head to the right = love & kisses
Sideways, head to the left = I promise not to leave you :-)
Diagonal, towards the right = Won't you marry me?
Diagonal, towards the left = Yes, I will
Another set of definitions:
Stamp Positions & Meaning
Upside down, top left corner = I love you
Crosswise on top left corner = My heart is another's
Centre of envelope, at top = Yes
Center of envelope, at bottom = No
Straight up and down, any position = Goodbye sweetheart
Upside down, top right corner = Write no more
At right angle, top right corner = Do you love me?
At right angle, top left corner = I hate you
Upright top right corner = I desire your friendship
Upright in line with surname = Accept my love
Upside down in line with surname = I am engaged
At right angle in line with the surname = I long to see you
Centered on right edge = Write immediately!
Actually, I remember using the upside down stamp to a girl when I was stationed overseas in 1968. I promptly got a Dear John letter in return.
