I think this mark, on Kennedy's forehead, was made after it was produced, and in circulation. But I have to check to see what you all think. Still kind of cool.
I have a rubber stamp that says "Postage rates are too high". I stamp all my one dollar bills with it for fun. Kind of like doing the "Where's George" thing and making a point.
Not sure what the D stands for but it's known as a countermark. This is considered damage as it was done after it left the Mint. Welcome to CT.
Compared to other countries our postal rates are very low. (For letters.) Packages are another matter. When the Postal Service collapses see how much it will cost to mail a letter. Probably $2-$3.