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.
But they are allowed to bring personal items on board. The error here was clerical. In fact some of the covers were listed under the personal items section.
So was that wrong? They opted to take those instead of other personal items.
Or, it demonstrates a lack of foresight by NASA to take advantage of an opportunity to sell a bunch of stamps over the following decades at wildly inflated prices.
There must be at least a handful of items that don't weight a lot per 1000 units that could have gone to the moon and back. I'm thinking pretty much any paper document or certificate, cash notes.
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.