Credits added in Sandbox account can be used for Purchasing subscriptions as well as making Paid API calls. You can read more about Paid API calls from here.
Credits can be added via the 'Billing Tab' on the API Dashboard.
The Current Balance Indicates the existing credits in your account.
1. To add new credits, you can drag the slider or enter the amount manually. 2. If you wish to obtain a GST invoice, check the "I want GST Invoice" box and enter the company details. 3. To purchase the credits, Click on 'Buy' button below. A Razorpay Payment Gateway popup will appear and you can complete the payment using multiple payment options. 4. Once the payment is complete, the purchased credits will be added to the current balance.
You can view the ledger for Credits purchased and Subscriptions bought under the 'Purchase History' in the Billing section.