as for now I figure out a way to make pay as you play,
I start a session with 6 or more hours and then when I end the session it said how much it used and charge for cost
but at least I need to get rid of the xx hours remaining on client when starting a session and the countdown from the server aplication or if there is a dedicated session start for pay as you play

generaly I need to start and stop a session to calculate the time and cost without the need to add time to be used

"Pay As You Play" = "Postpaid Session" or Not "Prepaid Session"

In Start Session window just clear "Prepaid checkbox" (which is blue when checked) and start pay-as-you-play session by pressing ok.
When user finish working, this session can be stopped by simply double clicking approprite computer (used time and traffic will be shown to confirm).

The are two options also :
Limited and Unlimited time.
If customers says somsing like "I dont know how much I will be playing, but no more one hour" and in you cafe used time reservation, the session can be limited to needed time. All that is needed - enter session length in Start Session window.
In other cases, unlimited session is general way to launch session without additional questions. Set length 00:00:00 for this.


Difference between "Postpaid Session" and "Prepaid Session + manula termination with money return" :
In first case, moneys are not added to the "cash desk" at session start, they are added only when finishing. In second - moneys are added immediately.

See also :
http://help.vaosoft.com/Server/AddTime.htm
http://help.vaosoft.com/Server/PostPaid.htm