There is no hidden code here, it just means he wasn't paying attention and could have been in a rush when he put the stamps on the envelope.
Nothing
In mathematics, an upside down A means "for all".
A upside down rainbow means cheese or tacos
I'm not sure exactly what you mean. I guess you could write the letters upside down. If you write the words upside down, the letters will be in reverse order and won't cross correctly. Most crosswords have 180 degree symmetry, so if you turn the puzzle upside down, you could enter the words but the numbers won't line up. The bottom line is you can do whatever you want, but you will have diminishing success in actually finishing the puzzle, which is mostly the point for me.
Answer it
The Upside down T symbol indicates perpendicular lines.
pnworhd is an upside down pyramid. If you take pnworhd in script and turn in upside down you will get pyramid
up
IgLoo
It means "stamp" - the stamps which are pasted on letters when you send them.
Anokatoprosopus means upside-down face (i.e. some aliens or monsters have upside-down faces).
It means un-polo if it's an upside down p with a small diamond next to it.
The upside down purple triangle was actually used to identify Jehovah's Witnessess