To purchase more credits:
Click your organization name in the top-right corner
Click Billing (as shown below)
Scroll down to click Buy credits at the bottom
Choose how many credits you wish to buy, and click to Buy now
You can choose to pay immediately with a credit or debit card, or to download an invoice.
If downloading an invoice, you can also pay by bank transfer. The credits will be added to your account once payment is received.