Answer: There are two main reasons given for the mitzva of mishloach manos. According to the Terumas Hadeshen (111) we do so in order to ensure that everyone has food for their seuda. The Chasam Sofer (OC 196) quotes R’ Shlomo Alkabetz (Manos Halevi 9) who writes that Haman described the Jewish people as a nation scattered and dispersed among the nations (Esther 3:8). By giving food gifts to others on Purim, we demonstrate our friendships.
Following this, there is a machlokes as to whether one can send mishloach manos before Purim to arrive on Purim. The Baer Heitev (695:7 quoting the Yad Aharon) writes that as they have received their food, one fulfils their obligation regardless when they sent it. The Aruch Hashulchan (OC 695:17), however, writes that as the primary reason is to increase happiness and friendship, one must give it on Purim, too.
The Ben Ish Chai (Torah Lishma 188) explains that this machlokes is dependent upon the machlokes as to why we give mishloach manos. The Rema (OC 695:4) writes that if one sent mishloach manos to another who refused to accept them, they have still fulfilled their mitzva. Clearly, the Rema follows the Manos Halevi that the main reason is to promote friendships. As one sending wouldn’t experience friendship on Purim by sending mishloach manos earlier, one shouldn’t do so (See Rivevos Ephraim 4:173:32).
Nonetheless, the Aruch Hashulchan (OC 695:16) writes that one can appoint a shaliach to deliver mishloach manos on Purim. Likewise, R’ Shlomo Zalman Auerbach (Halichos Shlomo, Purim 17:14) writes that one may pay a business to make up and deliver mishloach manos on Purim as they act as a shaliach.
In conclusion, while one may pay someone to deliver mishloach manos to another on Purim, one should not send a food parcel before Purim that will arrive on Purim.