Can a person mail a letter on friday as long as its mailed early enough for the post man to retrieve the letter even though the post might actually pick it up on Shabbos?
Answer:
There is no problem to post a letter on Friday, even just before Shabbos, because this does not imply an instruction to do any labor on Shabbos.
A different question would be if there would be an instruction for Shabbos delivery, which we have addressed in a different post.