"Forever" first-class stamps.
"Forever" first-class stamps. - 
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In the risky world of economics, there was always (thought to be) one guaranteed investment: forever stamps. 

That stamp that will let you send first-class mail, always and forever, until the universe pulls back on itself and shrinks everything into a black hole. They guaranteed that you could pay for them without worrying that they'd be worth less in the future. 

Now the Washington Post's Allan Sloan says that "for the first time in almost 100 years, it looks like the price of a first-class letter could drop." 

Beginning April 10, the price will likely dip from 49 cents to 47 cents. 

Turns out mail can yield life's great lessons. 

"The lesson I've learned is that just when you think you have figured something out, you realize that nothing is forever," Sloan said. 

Click the above audio player to hear the full interview. 

Follow David Brancaccio at @DavidBrancaccio