He [Uncle Vernon] held up the envelope in which Mrs. Weasley’s letter had come, and Harry had to fight down a laugh. Every bit of it was covered in stamps except for a square inch on the front, into which Mrs. Weasley had squeezed the Dursleys’ address in minute writing.
“She did put enough stamps on, then,” said Harry, trying to sound as though Mrs. Weasley’s was a mistake anyone could make.

Author: J.K. Rowling