Since 2007, applying postage for most mail has been extraordinarily easy. You buy one Forever Stamp and put it on. It doesn't matter if you bought the stamp five years ago. You don't have to think about what the current rate is. You don't have to see any numbers at all. It's just one letter, one stamp.
However this simple rule only applies for what the post office calls "standard-sized" letters. For most mail, that's fine, but there are a couple of rules that greeting cards often don't compl...