The only way I can see to have "Boxed" print on the Sales Order without adding a quantity is to type it into the Description field without adding anything into the Item Number field.  

I'm not sure whether a bare Description line will print on a Packing Slip or not, I believe they do.

But if you were to use an item Number linked to a G/L account, and record a zero price for some quantity, you'll be able to run a transaction report on how many orders were to be boxed, or how many boxes should have been used - (if it's ever important or interesting to know)