You can purchase more photo grading credits in two ways.
1) You can find the "Purchase more grading credits" option on your main "Photo Grading" page:
Or 2) you can purchase them by navigating to your Billing screen by:
Clicking the top right corner with your initials, and then selecting "Billing"
Both option 1 and 2 will bring you to the billing screen. You will then select "Purchase More Photo Grading Credits":
After you select the Purchase option, it will bring you to the available packages where you can purchase & check out:
Select a package you would like to buy, click the "BUY" button and checkout. Credits will be credited to your account immediately.