An often discussed topic is the sending and receiving of letters on erev Shabbat and Shabbat respectively. In the modern era the question became extremely important with the release of the each of the Harry Potter books and whether Jews could order them from online stores such as Amazon to receive them on Shabbat. The Talmud states:
אין משלחין איגרות ביד נכרי בערב שבת
“one may not send letters in the hand of a gentile on Shabbat eve”
This question was dealt with on the Hirhurim blog here. The link has further links to particular articles about the Harry Potter issues. I have copied a short section below which may be of interest.
“….Somewhat related is sending mail right before Shabbos. R. Moshe Feinstein (Iggeros Moshe, Orach Chaim vol. 3 no. 46) notes that many Jews work in the New York post office, including on Shabbos. When you put something in the mailbox on Friday, you are essentially giving it to Jews to transport on Shabbos. Therefore, R. Feinstein forbids sending mail late on Friday. R. Shlomo Zalman Braun (She’arim Metzuyanim Ba-Halakhah 73:5) disagrees. Because at least some gentiles work in the post office, we can assume that they, rather the Jews, will handle your letter. I once heard R. Mordechai Willig say that this entire debate no longer applies today, now that so few Jews work in the post office.
Somewhat related is sending express mail to arrive on Shabbos. You are asking a gentile (presumably) to deliver your package on specifically Shabbos. This is amirah le-nokhri, asking a gentile to perform labor on Shabbos for you. Therefore, R. Yehoshua Neuwirth (Shemiras Shabbos Ke-Hilkhasah 31:20) forbids sending express mail to arrive on Shabbos except in urgent situations. R. Braun (ibid.) and R. Gedaliah Felder (Yesodei Yeshurun vol. 3 p. 63) similarly permit sending express mail in urgent situations. Their logic is that you give the package to a gentile who gives it to another gentile to deliver. This additional layer of distance from the labor renders it permissible.”
And so the answer is…depends who you ask.