Empty a box when it is full

To explain, I'd like my robot to pick up the letter from the end of the conveyor belt and place it in the "Queue2" bin. The maximum number of letters in the bin is 60. At the same time, bins from "Source2" arrive on the roller conveyor. I'd like the bins to be waiting in the middle of the conveyor, and when "Queue2" is full, the 60 letters are placed in a bin.

But unfortunately, I have no basis in process flow.