A detailed real-time overview of your usage per survey project can be found under your account.
They can be used for more than just surveys, respondents and mails. They can also be used to pay for services, such as custom design work, consulting, panels, etc. You can choose how many credits to buy with a minimum of 50. Before doing so you would do well to estimate your planned expenditures to ensure that your purchase is in proportion with them. You can request a tender to help you estimate your costs.
You will receive a free email notification that your credit balance is low when your credit balance falls under 10% of the total credit supply after your last purchase. If you wish, you may then purchase additional credits.