"I saw your reply"= "at sometime in the past I saw your reply". "I have just seen your reply" = "A few moments ago your reply has come before my eyes". "I just saw..." is incorrect in BrE.
I just saw your reply. And in AmE this would be a very common thing to say, especially when we are perusing our emails. It is equivalent to saying, "I saw your reply just now." Would that also be incorrect in BrE?
We would not, by the way, say, "I just see your reply." The word "just" places it in the recent past and "see" implies the present. One might say, "I just see your one reply, not the two replies that you say you have sent." In that case "just" means "only." I wonder if this is the same in BrE.