Does Costco's personnel have the right to open my bag?

I went to Costco shopping with my daughter, so put my daughter's diaper bag in the shopping cart.
Usually before customers exit from the door, there's some Costco's personnel checking for your receipt and goods you buy in the shopping cart.
But this time the guy asked me to open my daughter's diaper bag to ckeck on it.
Do I have the right to refuse to do so? Is it normal and comply to store policy?
Thanks!