Email them right now, don't put it off. Mention at the bottom of your email that you will also send them and hard copy and fax it to them or snail mail it to them. Don't panic, though, sometimes they are waiting for a few more letters often with an opposing view point on the same topic, so they might hold yours a few days and in some cases, they have even held letters for a few weeks. Well, they published your brilliant letter, every thing is fine. Save your ammunition as a surprise.
Every paper is different.
It goes back to psychology. There used to be two dailies in a major city, and if you wrote the identical letter to each, the one wouldn't print it. Word it totally differently and, both would print your letter on the exact, same thing. Your email might make a differance in whether some else's great letter gets published or gets tossed. Email your opinion in that case.