Quote:
So add a 31 cent stamp to them and they can be used just like post cards.
Not quite. Postcards/Postal Cards/Stamped Cards are currently $0.33 to mail. Your $0.15 postal card would only need an additional $0.18 to mail at today's rate.
Keep in mind that modern versions of postal cards in mint condition generally command little premium above the face value of the postal cards themselves. The used copies are actually worth more, but the key is that the used postal card must have a contemporaneous postmark to command those higher prices. In other words, you can't attempt to mail to yourself today a postal card from 1970 or 1980 and expect that the cancelled card would be worth a premium. If the postal card was from the 1970 or 1980 period, the postmark would have to show a date in that time period in order to reflect that premium value.