People buy stamps that are guaranteed to rise at less than the rate of inflation as an investment? And the USPS advertises it that way?? I'm not convinced. I w

People buy stamps that are guaranteed to rise at less than the rate of inflation as an investment? And the USPS advertises it that way?? I'm not convinced. I would need some evidence of that. (I am aware that stamps may be used in prison as a currency, but that's a special case.)