asking a favor of Lovewind, Xiaobaitu,or other warmhearted perso

I have a question, and expect experts in this forum can answer it. On the bottom line of the RFE letter from INS, it says "PLEASE USE THE ENCLOSED ENVELOPE TO MAIL ADDITIONAL EVIDENCE REQUEST BACK TO THIS OFFICE." This letter was forwarded by my lawyer and I did not see the enclosed envelope. My question is "should I have to use the "ENCLOSED ENVELOPE'? Can I mail additional evidence using the post office envelope? Thank you so much for your answer.