Drag and Drop Stacking Problem

Hello, Learning Support.

Thank you for reaching out!

What you're trying to accomplish would have to be done with a custom design (including states and triggers, as you mentioned), as I'm not aware of an out-of-the-box solution.

One idea I had, but it won't perfectly fit your design since your boxes have different heights, is to only allow the user to drop the package on one transparent shape at a time (from the bottom to the top):

This was created using motion paths that only move the next "transparent shape" into position when the one below has an item:

The only downside of doing it this way with your design is that if the user incorrectly places the smaller box first, then there will be a gap between the first box and the second. 

I hope others in the community can give you additional suggestions, or that this at least gives you a starting point.