What do you put in the upper right corner of a letter before you send it?

Explanation

A postage stamp is a small 'sticker' made out of paper and with some sort of illustration. Stamps are sold by the post and are used as payment for sending letters, postcards or parcels. Before the letter is sent, the postage stamp is stamped by the postal administration, indicating the date and location, so that it cannot be reused. Many people collect stamps. Rare and particularly sought after stamps can be worth a lot of money.