“J,” in north Africa, recommended…
http://www.electronicdonations.com
I noted that they do donation by text (although it appeared that users had to register via a computer first, using their web browser). In addition, we’ve used Good Shepherd’s bank for some time…
http://www.goodshepherdsbank.com/
But maybe you have a favorite?
A little different type of donation, but thought I’d put in a plug for forafriend.com for small, relational, personal gifts to missionaries.