Why does the nurse mark the medicine down as charity?

Although she's dealt with Phoenix on a number of previous occasions, the nurse still looks at her as just a frail, little old lady, one of many who turn up to the clinic on a regular basis. To the nurse, Phoenix is just "Aunt Phoenix," a slightly bothersome granny who turns up a couple of times each year to pick up some medicine for her grandson. The nurse marks the medicine down as charity as she assumes that Phoenix is poor, a charity case, so to speak. The attendant has the same impression, which she confirms by giving Phoenix a nickel from her purse, as it's nearly Christmas.
Phoenix says that she'll use the money (plus the nickel she picked up from the hunter earlier in the story) to buy her grandson a paper windmill. Despite what Phoenix says, we're not actually sure that her grandson really is still alive. If he has indeed passed away, then Phoenix's acceptance of both the medicine and the money on his behalf, and not hers, is a way of maintaining her pride and dignity in extreme old age.