He's rather visibly surprised when she offers to have him pick something out for her, as it really wasn't something he was expecting at all. "Ye...what?"

Jamie's first impulse is to ask her why on Earth she would want him to do that. Unfortunately, his second impulse is pretty much the same as the first impulse...so he goes ahead and blurts it out.

"Why would ye want me to? I've not picked out an outfit for a girl before. Are ye sure ye want to trust me with doing that for ye?"