I think the tech was more worried about penetration rather than a few dollars. It's been proven that 2 blades penetrate farther than a 3 or 4 blade. Like Starr says, if she's not getting the penetration, then switch.
I will tell you that those Stingers are wicked! I shot one a while back, missed my target and it went through heavy metal siding and into a couple of layers of sheet metal that was sitting outside...had a heck of a time getting my arrow unstuck!