I have been penpalling for decades. Although I learned the proper way to write a letter in grade school, until recently, I never included a return address on my letter. I thought it was a bit too much, given that I’m quite religious about always including the return address on the envelope. I figured, My new penpal doesn’t need it in both places.
But experience has taught me what logic did not. Including your return address inside the envelope is insurance. If something happens and your envelope is mangled or doesn’t arrive in the pristine conditions in which you sent it, the return address on the letter will insure that the penpal can write you back.
There have been times when I could not read the return address on the envelope, because the post office has put their mark at just that point. And then I am stuck. I have a lovely letter from a potential new penpal but I cannot respond to it. Not a good way to win friends and influence people.
I encourage you to put a return address on your letter, as well as on the envelope. (The latter is more for you – in case your letter doesn’t reach your intended recipient, it will come back to you.) I would even encourage you to include your return address in each letter. Doing so makes it easier for the penpal, rather than having to look up your address when they reply. The easier it is for someone to respond, the more likely they will do so in a timely manner.
Keep calm and write on!