If you have credits available on your account, you don't need to perform any special actions to use them. When you login to your account and prepare an order (see @4.3.1. Purchasing tests for the desktop version‍), the system will automatically use up credits on your account if available, before asking for you to pay for tests with a credit card.

Therefore, if you have sufficient credits on your account, the basket total in dollars will show as $0.00 and you will be able to complete the order without making any further payment.

If you don't have any credits, then you will be able to pay with Paypal / credit card for your order.

If you have some credits in your account, but not enough to purchase all the licenses in your order, the credits you do have will be used first, and any remaining licenses will be charged at the prevailing rate.