This is literally the first time I have ever used an Arduino or tried to do coding, so I don't really understand what I am doing...
wvmarle:
You stated before that every tick is 10 cents. Then why adding just 1 cent per pulse?
Thank you, I've changed it to 10 cent per pulse.
wvmarle:
Also, do you really need to use interrupts? Get rid of that delay(1000) in your code, have a look at "blink without delay" on how to do regular actions without blocking your script, and you can just start to poll the coin acceptor to see whether a pulse is coming in (you will have to add some code to make sure you count each pulse once and only once, only if pulses are extremely short or you have a really heavy workload elsewhere you need interrupts for this).
As I don't know what I am doing, I have no idea why the person that wrote the code used interrupts, or what they even are 
I've just looked at "blink without delay" and have no idea how to change the code to remove the interrupts.