The chapter states that 'A figure of an envelope or stamp is not needed but the address can be written. If any student provides them, it will not earn them any extra credit or penalty.' This indicates it's not an essential part, unlike the other options which are listed as essential parts.
Class 6 › English Grammar and Composition › Chapter 9: Letters & E-mails › Topic: Layout Styles and Guidelines for Formal Letters
Aligned to the NCTB national curriculum.