Question: Am I allowed to order something online to arrive on Shabbos?
Answer: The Mishna (Shabbos 121a) writes that one mustn’t ask a non-Jewish person to extinguish a fire on Shabbos. Rambam (Shabbos 6:1) writes that this prohibition (amira leakum) is miderabanan (See Shaar Hatziyun 243:7).
The Shulchan Aruch (OC 244:1) writes that one may pay a non-Jewish person to do a melacha for them if they don’t specify that it needs doing on Shabbos. Thus, one may give one’s car in to a garage on Friday and pick it up after Shabbos providing they have enough time to do it before or after Shabbos if they want. The Mishna Berura (244:24) notes that this must include daytime hours when it normal for one to work.
Thus, the Shulchan Aruch (OC 247:1) allows one to pay a delivery man to deliver a parcel without worrying when they’ll deliver it, providing that they weren’t specifically instructed to deliver it on Shabbos. Likewise, the Mishna Berura (247:4) writes that if they have been instructed to deliver the parcel on a particular weekday, they must have enough time to travel there without travelling on Shabbos.
Nonetheless, the poskim write that there are a few reasons to be lenient to allow sending a parcel that will arrive on Shabbos.
Firstly, R’ Mordechai Yaakov Breish (Chelkas Yaakov 1:65) and others write that as the deliveryman is delivering so many other parcels along with this one, they are not performing melacha especially for the Jew. The Mishna Berura (318:13; Shaar Hatziyun 316:33), however, writes that this may still be problematic mideoraisa.
Additionally, the Chavos Yair (53) writes that while one mustn’t instruct a non-Jewish person to perform melacha on their behalf, one may ask them to instruct a second person to do so (amira leamira). According to the Chasam Sofer (OC 60) this is especially true when the instructions were given before Shabbos. The Mishna Berura (307:24; Biur Halacha 307:2) writes that one can only do so to avoid a major financial loss.
Based on this (and other reasons), R’ Pesach Eliyahu Falk (Machazeh Eliyahu 37) says that there is no issue in sending a package to arrive on Shabbos.
The consensus of poskim (Minchas Yitzchak 6:18; Teshuvos Vehanhagos 1:278:2; Shemiras Shabbos Kehilchasa 31:20), however, is not to do unless there is an urgent necessity.
In conclusion, one should avoid ordering something to specifically arrive on Shabbos unless one really needs it then.