Yep it's quite simple once you understand the basics.
You just need some waypoint code and then some variables to act as toggles between States and to check the different conditions (i.e. food is there, quality of food etc).
So something like this
If state = enter
Waypoint code to walk inside.
Check near to counter or seat etc.
Give order info on screen.
Change state to waiting to be served.
ElseIf state = waiting
If order is ready
give player money.
Change state to leaving.
If waited too long
Change state to leaving.
ElseIf state = leaving
Remove order from game so can't be reused for other ai (displayed request and any actual game object used to show order ).
If reached end of waypoints then delete this AI.
As you can see, just break your scripts down to logical states or conditions and it will make it much easier to control what is happening and when.
You'll want other scripts to manage stuff like table/food quality and if an order is ready or not... But that's the good thing about object oriented programming, it's super easy to make many scripts that relate to only certain objects.