First Class Stamp

So the Postal Service is raising their prices again. You'd think they would know about these things a few days before the change went into effect, though. 12 sided wife bought a hundred roll of 37 cent stamps not 3 days ago, only to find out that in less than a week they'd be out of date. One would hope that someone at the Post Office would say to her, "hey, you might want to buy a hundred roll of 2 cent stamps while you're here, because the price is going to 39 cents on Jan. 9th."

I don't understand why they don't just create a single stamp, put a giant "F" on it for first class, and charge whatever they charge. Anytime the post office saw a stamp with an "F," it'd be all good. But how would the Post Office know what was paid for these "F" stamps? Wouldn't people would just stock up on cheaper stamps, you say? And this would be a bad thing? Au contraire, the time value of money being what it is, it far more desirable to have large amounts of cash right now. Who knows, it might be the biggest boon in the history of the USPS.