Use Google Docs, I assume you can use it on Safari on a Mac, or hopefully Google Chrome itself.
"a letter". I'm on a Mac and not using
Use a magic marker!
Use online alternatives.
Apple, Google & Microsoft all have online versions of their word processors available to use
Download Libre Office. It's free and includes a word processor that's very similar to MS Word. It can actually read and save files in MS Office format too.
Lay the screen down flat, put a piece of paper on it. Write on the paper with a pencil.
"a letter". I'm on a Mac and not using a word processor.
Use Text Edit. It can be found in your applications.
Pages. It's free. Should already be included with the OS. If not get it free from the App Store.
Expect postage costs to be high.