Should I tell my manager?

Let me rephrase: "Should I ask to expand my duties and do something I think I'd enjoy or should I just let someone else dictate my life for me?"

It sounds like they put you in a position that you're really good at - why complain?  I guess you could discuss this, with your manager.

Nothing ventured, nothing gained. Talk to him.

You could ask your manager for additional responsibilities if you want, but if you're very good at what you're doing, the manager may not want to move you. It's worth a try, though. Just keep your expectations low and don't feel resentful if they say no.

You can ask but don't expect them to change it. They like that you're fast with being a cashier. Also it can be tough trying to train someone that's new to being a cashier, and pushing them to go faster. You would have to ask, and convince them to let you bag the foo instead. 

is it your money out of your pocket to pay for cashback , or is the manager worried , no , mcdonalds can afford it , you should be worried about your lousy pay .. 

If you don't bug them to move you then you will never be promoted because you have to know everything for that, if you want to climb then you will have to say something, if you don't want to do that then there isn't a need unless you just don't like it where you are. Also to consider, I was put on the grill, got really good at it, then they changed the layout of the store to "experience of the future" and I lost what I was good at and didn't know anything else. They don't change layout often but they could any time.

You can ask you have nothing to loose

Let me rephrase:  "Should I ask to expand my duties and do something I think I'd enjoy or should I just let someone else dictate my life for me?"  That's essentially what you're asking.  For me, that answer to that question is clear.  If you don't like the answer, that's a different story.  There should be no harm in asking though.

If you are doing a good job, no reason to change...so to speak.