This would be more like the President renting out a room in the White House. I think the line is actually pretty clear; if you're using state resources to produce something of value to sell for private profit, you're on the wrong side of it.
While 400 stamps don't weigh a lot, they'd still cost more than $1 each to launch by my back of the envelope calculations. You're effectively footing NASA with a $500 bill to pocket $21k.