There are various charities that advertise the possibility of donating to their charity and receiving gift cards to give in lieu of Mishlochei Manot- will I fulfil the Mitzva if I only send these cards?

As known there is the Mitzvah of Mishloach Manot and giving charity to the poor. The Mitzvah of Mishloach Manot can be fulfilled only with two ready to eat foods (Shulchan Aruch 695:4). One does not fulfill the Mitzvah by giving charity instead. (Trumat Hadeshen 111).Therefore, by sending cards only, you will not be fulfilling the Mitzvah. On the other hand, the Rambam in Hilchot Megilla (2:17) writes that it is better to give more Matanot La'evyonim than to spend on the Se'uda and Mishloach Manot. So if the arrangement is that you spend a little money on the Mishloach Manot and spend more money for the poor which will be given on Purim you will fulfill two Mitzvot at once and you will be doing exactly what the Rambam prescribed. Have a happy Purim!