Whenever I call a certain friend of mine on the phone, I usually say, "Hey, Chuck, this is Don," and he says, "Hey, good buddy!" To me, his use of the word good with buddy--not to mention the pleasant tone of his voice-- makes all the difference to me, as it may with your online friend. I cannot say the same, however, about "good pal," "good bro'," "good mate," or "good dude." I don't know, they just don't sound right!
By the way, the nonverbal aspects of spoken communication, such as tone of voice, do not "translate" so well in written communication. This may sound patently obvious, but it's still worth considering when that little voice inside of you gives you pause, and you ask yourself, "Should I use this expression or not?" It sounds to me as if you might have some doubts about using the expression "Thanks a lot, pal!" with your online friend. In spoken communication I've heard that locution used in an ironic, even sarcastic, tone too many times for it to sound--to me, anyway--perfectly innocent or friendly in written communication.