It's not so easy to identify an associated actor, however. No particular user role requests a bill. The bill is sent to the customer, but the customer wouldn't object if it didn't happen. The best guess at an actor here is the Billing Department, in that it gets value from the use case. But Billing is not usually involved in playing out the use case.
Actors can have various roles with regard to a use case.
They can be the ones that get value from the use case, or they can just participate in the use case. Depending on how you use the actor relationship, different actor roles will be important to you. I tend to be most concerned with controlling system development. The key is to remain aware that some use cases will not pop out as a result of the process of thinking about the use cases for each actor. If that happens, don't worry too much.