Summary: 

To purchase an API bundle to enable you to allow more API calls than the standard limit on your plan. 

Steps:

1. Navigate to Settings
2. Click Billing
3. Next to Add-ons, click Modify
4. Under Extra API Bundle, click Add
5. Click Upgrade  

Additional Information:

For more information on the pricing click here and under What's included in each plan, scroll down to API Access and click on See extra bundle pricing