She just sounds quite shy. I think that's all and needs to practise relating to people. Don't push her to join in, as that puts pressure on. Just be there, without a fuss and let her join in in her own time. For shoe shopping, or other things involving strangers, try a bit of bribery. "if you are good in this shop, I'll put a point on your chart" and let her choose a small treat when she has a certain number of points. At this stage, just behaving in one shop is fine, as it's all building her confidence, until it becomes a habit.