It is ideal to pay it when your lodge your application, otherwise you have to wait for their instruction as they will let you know of the payment instruction and their it part of the mail that you to send back with your receipt.
I have the same issue too, my consultant advise me not to declare my spouse and daughter due to limited funds which I agreed. I have not consulted a lawyer as they are too expensive as I am only working in a factory and on EI on several occassions... making it very difficult to sponsor them...