Walmart will send you a Ready for Pickup notification on your account or application when your order is ready and their store is open from 8 am-8 pm.
You should check in to the store through your app when heading towards Walmart.
This way they will get a heads-up and after entering your parking number, an employee will come and load your order in the car within 2 minutes, that way it will be faster for you, hence saving your time.