Payment for ad budgets | App

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We are an ad agency and, what our app is, is this. We have a flat fee, but the actual product is that we help store owners advertise, and so they need to send us their money.
But, in order for them to send us their advertising budget, let's say its $100 a month, up to them, we want to make it as easy as possible for the store owner to send us his ad budget, so ideally, they authorize the charges, and so $100 a month is taken from their deposit and sent to their account with us. We then spend that money on ads. 
If someone could tell me how this charge could be made possible, that would really help us out a lot!