Regardless the payment method you are using you can choose to selectively lock or unlock pay before download on each Order. You can toggle this option from the Order Overview Page:
If you Toggle On - Force Payment the agent will be forced to pay any outstanding charges on this order prior to download. They will see a watermarked preview of the images prior to payment
If you Toggle Off - Force Payment the client will not have to pay before download and the order charges will be sent in a weekly invoice.