Once you've reached the top of the Glacier zone, you'll come across the final shop of the zone. This shop has no shopkeeper inside, but interact with the cabinet anyways. You will need to interact with it a total of 47 times before he begins his lecture which is much, much longer than the first one.
Here is the location of the final shop in the Glacier zone.
Lecture 1 of 3.
Lecture 3 of 3.