Presumably you already have some code to take what the user types in and turn it into a number. What does that code look like?
What happens right now if you type "foo" in when you are asked for amount due?
An Exception?
Then you need to catch that exception and "handle" it in some way.
One way of handling it is to set up a loop until the user enters in a "valid" value.
A similar approach could be taken for insufficient amount tendered.
You should be able to calculate if they have given you enough money to cover the bill.
If they haven't then keep asking until they do.