Q. I a friend that I want to send shalach monos to is going to be out of town during Purim, can I send to him the shaalch monos before Purim, before he travels and tell him to have in mind to acquire them only on Purim?

A. Beir Heitev (675: 7) deals with a similar case when the shalach monos were send before Purim and arrived on Purim. He quotes Yad Aharon that one complies. However Piskey Teshuvos (ibid 21) mentions that many Poskim maintain that they should be send and received on Purim itself. See also Nitey Gavriel 29: 5: n. 7) that when one has in mind to give the matonos only during Purim it may be better.
Horav Shlomo Miller’s Shlit”a opinion is that if one will comply properly with the mitzva of shalach monos on Purim day, he could send when needed, additional matonos before Purim

Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit”a.