Good luck!! If there is a toilet in store, I would take her before starting and also leave my shopping at customer services halfway round to take her again!! I did this when my (now 20 year old) DD and she never had an accident in there.
Going to do the weekly shop today, DD been in knickers for just under 1 week but obviously still having some accidents. Gong to make her go before we leave to take DS to school then onto the shop, with lots of spare clothes!