Is there a boolean situation in use case diagram? If not this else do this? here is my example:
The user doesn't need to know that we create him a user, all happens in the background. So it's like a client application that communicate with another server. If the user already exists in the new server, just process the add to cart, otherwise, create a user for that specific user on the other server.
EDIT:
Thanks to Thomas Kilian advice's, this should be the right answer: