AITA for not buying the exact chocolate my girlfriend asked for?

My girlfriend is at home sick and asked if I could go to the store to buy her a couple of small chocolate bars, I told her that that would be no problem but asked why I shouldn't just buy one chocolate plate instead (same brand, same chocolate just a little bit thicker and cheaper) and she said no she wanted five small bars. She gave me $40 in cash and I went to the store. When I got there and saw the price difference I just couldn't justify paying more for less chocolate so I bought one small bar and one large plate. When I came home and explained this to her she got a little upset both because she feels like I don't listen and also because she wanted the small bars, but I don't get it since it's the same chocolate just not as thin, but it tastes the same, it's cheaper and it's just chocolate...? So is she overreacting or am I the asshole?