Winry works to make her automail as affordable as she possibly can, especially because the majority of people who need it are manual laborers and traumatized war survivors, but she charges Ed as much as possible because the military pays for basically everything he has and she'll jump at any chance to drain the government's coffers.
She gets a sadistic sense of satisfaction putting everything she can think of on the bill that Ed doesn't even look at and that Roy signs off on without reading.
Roy: Winry charged you 60,000 cens for three water cups.
Ed: That's actually charging for the water in the cups. She just gave me a glass from the kitchen and kept filling it.
Roy: And the 15,000 cen pillow fluffing fee?
Ed: Yeah, I don't know what to tell you, I think she just adds things at random.
Roy: Wow. I really admire her determination to drain our government's money. Your payment request has been approved.
