When I have something to mail, I usually weigh the item on our digital kitchen scale, and then calculate the postage on the USPS website. Then I either use whatever stamps we have around the house, or I tuck a check into the mailbox along with the item to be mailed.
Yesterday, I left a padded envelope for the mailman. It contained a little gift box of baby clothes for my brother's newest sprog. Total weight: fifteen ounces. The postage for First-Class Mail was $10.40, and I just happened to have a book of ninety-cent stamps around, so I plastered the front of the envelope with little pictures adding up to the required amount.
Today, the mailman brought the whole thing back. It wasn't because of a lack of postage, but rather because of a new directive from the Heimatsicherheit Ministry. Apparently, there's a new rule in place that requires anyone mailing a stamped overseas package weighing more than ten ounces to drop it off personally at the post office. This will presumably keep terrorists from sending Semtex through the mail anonymously, although the nice (and apologetic) postal employee was unable to tell me how exactly this is going to be accomplished. Mind you, the rule applies only to stamped mail, not metered.
So now I have to keep my gift packages under ten ounces, lest I have to saddle up the battle wagon and drag both kids to the post office with me, requiring a complex logistical ballet and an hour of my time to mail a frakkin' box of baby clothes.