While we await Tina's clarification, if we continue to assume she means the highest face denomination POSTAGE stamp, the highest value I know of is the 500 quadrillion (500,000,000,000,000,000) pengo stamp from Hungary. It is Scott #774 shown below.

I know that it says "billio" on the stamp -- but the number convention used in Europe at that time, billion was the equivalent of the US "trillion". The correct Central Europe vs US equivalents at that time were:
Europe -- US
million -- million
milliard -- billion
billion -- trillion
It beats the German inflationary stamps. But the way things are going in Zimbabwe...

k