These days, if I am actually mailing a letter through the post, I make an educated guess about the weight of it and stick an extra stamp on or two for good measure. After all, stamps cost less than a dollar each .. I use the kind that never expire, so I'm not even sure how much they cost individually any more. I buy them at the ATM on sheets the size of twenty dollar bills.
But in my youth, which after all was quite a while ago but not so long ago as all that, it was ever so important that one never put on even an extra penny's worth of postage. We had postage scales at home, or always conferred with the grumpy people at the post office. If you accidentally sent a letter that lacked a penny or two in postage, the letter would arrive "C.O.D." and the mailman would ask the recipient to pay up. There was actually a form for this. I remember taping pennies to the form and handing to the carrier.We cared about pennies! Waste not, want not!