Going postal

The invites are in the mail to 20-some guests. We entrusted them to the Wicker Park post office, which is notorious for its horrible service. Example? A Valentine's Day card from my grandmother was delivered on St. Patrick's Day. Seriously. But we're hoping since we used the outside post box, the letters will never touch the hands of an incompetent postal clerk. An irrational worry: did they need extra postage? Bad form to deliver an invite with a notice that you owe 10 cents. Egh.