USPS Charging To Hand Cancel Covers & Where Can A Stamp Go?

Tomorrow is my junior stamp club meeting and I dreamed up this idea where the kids would color (or draw their own if they want to) four covers that when put together would make a picture. Here's my example:



We are going to use the new Santa stamps and put one in each of the outer corners. Then I was going to send them off to be hand cancelled at North Pole, Alaska.

Sooooo, I asked at the post office if it was ok to put stamps in the bottom corners and was told, "No!" Then I was told that it would be a 20 cent per cover hand cancelling fee. I had never heard of this before.

Any insight into these two problems would be most welcome:
1. Can you put a stamp in a different corner if it's going to be handstamped?

2. When did the USPS start charging to hand cancel?

Thank you!
Aimee