If I needed ten of something but bought a case of dozen and sold two of them at or below cost, I saved money as long as price per item on the ten pieces I needed ends up being less. That bigger company buys more from subs so they're probably able to negotiate better pricing.
If the additional hassle increases work load on your office your cost didn't go up. Also, if you sub something out to them at cost, it keeps their gears turning even if you made no markup. If they utilize you sometime down the road, you benefit from that too.