The paper is quite sturdy. Don't fret much about flattening it once for scanning, etc. Just don't unfold and refold it a bunch of times. In fact, if the content is interesting, I often leave them unfolded to continue to enjoy them without additional wear and tear.
Thank y'all for the prepaid vs. collect business when it comes to early stampless postage/mail. That in itself is pretty neat -- send a letter COD and stick the other person with the cost. Pretty fun :)
Still, 10 cents in 1851 was some serious business ...
collect vs prepaid: your letter is going back to the headquarters of the American Home Missionary Society. What easier way for national organization to pay for the postage expenses of their men in the field than to have everything come back "collect" for central payment.