Perhaps if the Postal Service made inflationary stamps that are always guaranteed to mail a letter, though, the price for them might change...
Like, I buy a stamp now for 37 cents. But, the same stamp will sell for 41 cents in a few months. So, I can use the first stamp I bought at 37 cents just like a newer 41cent stamp. That way, the post office would get a ton of people stocking up on stamps and there would be an influx of new money into the office. Then, the new money could be used for investment purposes and streamlining the business model so the post office could be profitable. Stamps could be like 'mini loans' to the post office. And, the people that buy stamps would like it because they would never have to worry about rate increases as long as they had stamps.