I showed before how to pass a parameter on the URL when invoking a taskflow (where the taskflow starts with a method call and then a page).
However in some simpler scenarios you don't actually need a full blown taskflow.
Instead you can use page level parameters defined for your page in the adfc-config.xml file.

So below is a demo of this technique.
I'm also taking advantage of this video to show the concept of a view object level service method and how to invoke it from your page.

P.S.
You might wonder - why not just reference #{param.amount} as the value set for the method parameter? Why do I need to copy it into a viewScope parameter?
The advantage of placing the value in the viewScope is that it is available even when the page went through several sumbits. For example if you switch the "partialSumbit" property of the "Next" button to false in the above example - the minute that you press the button to go to the next department - the param.amount value is gone.
However the ViewScope is still there as long as you stay on this page.