What kinds of invitations require additional postage?
Square envelopes, some box mailers and invitations that are heavier than one ounce require additional postage. Using an enclosure around your invitation like a petal enclosure or folder may cause your invitation to exceed the weight limits for a first class stamp. Take your proof or an extra invitation to the post office and ask them to weigh it for you before you purchase your stamps to make sure you have the correct postage.