Is it permissible to send mail on Friday afternoon, knowing that the postal workers will be transporting and/or delivering the mail on Shabbat? Seemingly, sending mail late in the day on Friday violates the Halachic prohibition against instructing a gentile to perform Melacha (forbidden activity) on ones behalf on Shabbat, and should be forbidden.
The Shulhan Aruch addresses this question in Orah Haim (247), where he writes that one may send mail on Friday, on two conditions. First, the price for the service must be a fixed amount, as opposed to an hourly rate, and, secondly, the Jew may not specifically ask that the mail be delivered on Shabbat. In the case of the modern day postal service, both these conditions are met. The postage is a fixed amount, and one does not specifically request that the postal workers deliver the mail on Shabbat. It is their decision to do the work during Shabbat, and therefore even though one knows for certain that the mail will be delivered on Shabbat, he may mail the letter.
A more complex question arises concerning sending express mail on Friday. If a person delivers a package through express mail on Friday, with specific instructions that the package must reach its destination by Monday morning, then he in effect instructs the gentile workers to transport the item on Shabbat. Since he specifically requests that the package arrive by Monday, which cannot occur without Melacha being performed on Shabbat, he is considered as though he explicitly requests that Melacha be performed on his behalf on Shabbat. Accordingly, Hacham Ben Sion Abba Shaul (Israel, 1923-1998) ruled that one should not send express mail on Friday, since this effectively amounts to giving specific instructions that Melacha be performed on Shabbat.
However, Hacham Ovadia Yosef, as recorded in Yalkut Yosef, ruled that in situations of urgent necessity, one may be lenient in this regard. When a person brings a package to the express mail office, he does not speak directly to the deliveryman who will transport the package on Shabbat. The Jew speaks to the office clerk, who then relays the instructions to the delivery personnel. As such, according to some views, sending express mail does not violate the prohibition against asking a gentile to perform Melacha on Shabbat on ones behalf. Hacham Ovadia thus rules that in situations of dire necessity, when a person urgently needs a package to arrive somewhere by Monday, he may rely upon the lenient position and send the package with express mail on Friday.
Summary: It is permissible to send ordinary mail on Friday afternoon, even though it will be transported or delivered on Shabbat. One should not, however, send express mail on Friday, unless this is urgently necessary.
