It depends on what add-on services you select. Advertisement on paid job sites varies from 1 to 2 job credits consumed depending on the site. Also, 2 job credits will be consumed when selecting the standard Headhunting service.
You can buy job credits from within the system and pay online or contact our sales team for offline payment. And yes, you get discount when buying a bundle of credits and you can choose when and where to consume your credits.