My system is a soda can machine. You can put a coin and buy soda from this machine. The interface is a keypad and digital screen.
One of my requirements is that the user can put x dollars into machine and then add items to his order. Then, the machine will show him the order total.
That is, "show user order total" Should I show this in my use case diagram? If yes, then how do i show it?
No, you should not. For several reasons.
If you still want to highlight that aspect in your use case diagram (which can be useful depending on your needs) then consider naming the use case "verify order total" and assign it to your primary actor. IMO that would be perfectly fine for a subfunction level.