I would add that even if you just add a PURCHASE ORDER first. Then proceed to receive items + bill it will mark the billable checkbox. (So no sales order, etc.)
All they have to do is add a link on the column label 'billable' to allow you to select all and de-select all by one click. That's all. Easy to solve on the spot in the meantime. Otherwise you have to click 1 by one and with many items -- a pain.
