How do I get through the stone wall

Hint 1 of 7:

You'd need to be beastly strong to break that wall.

Hint 2 of 7:

And well motivated, too!

Hint 3 of 7:

Did you pick up that mail order form from the space station locker?

Hint 4 of 7:

Did you mail it at the box in the forest near the hovercraft?

Hint 5 of 7:

Did you pick up the whistle?