It looks like you understand the problem to be solved. The next step would be to decide how to solve the problem. So now is not the time to worry about needing if else statements or "choice = cash". But while we are on the topic of "choice = cash" be sure to use == to compare instead of =, and use .equals with strings.
So without thinking about code, write down how you will be able to solve the problem.
Then go back over the solution to the problem and make sure it solves the problem without breaking any of the rules.
Once you get that done, all you have to do is translate to code. That will determine when/if you need if/else statements and if "choice == cash" or not