I wouldn't really consider them an expense in the same way as labour costs. Maybe in terms of the costs of distribution and storage for things that aren't moving. Things that are moving though with a profit margin aren't really a cost center.
In bookkeeping, no, but in looking for opportunities for efficiencies, it's pretty relevant.
Say, you've got a single employee on a $60k salary moving goods worth $10mm, but at a razor thin margin, so your profit on the goods is $100k, and your net profit is $40k. If you want to increase your net profit, it would probably be easier and yield a bigger return to see if you can do something about your $9.9mm product expense than your $60k salary expense.
