Now why would the United States Postal Service do this? There's nothing that small printed on any other stamp they make. Is it an accident? An oversight?
No freakin' way. There's no doubt in my mind that it's premeditated. They're hoping that some people (the elderly, perhaps, you scheming bastards?) will buy the stamps, and then, sometime later, reach for a one CENT stamp, see the 1 but not the minuscule $ sign, and put a $1 stamp on their envelope instead.
Great way to make extra money, USPS. Capitalizing on the elderly or confused is a delightful way to fund your operations.
