re: Walmart Grocery curb service- when do you pay?

It’s a great service

Find out from her when the pickup will be most convenient (which hour).
When you place the order, you'll first set up your account and in that will be the charge card that will do the paying.
You'll also indicate the name of the person (Susie C.) who will be doing the pickup.
This will give her an additional hour of the day and she won't have to wrangle the child out of the car, etc.
Also there is a place where you can opt out of substitutions if a specific thing is out of stock.

We've been using this convenience for about two months (we tend to use it every two weeks) and have excellent service.