Credits allow you to lock in a volume discount, simplify your expense reporting, and introduce more predictability into your geocoding bills.
Credits never expire and they rollover month-to-month. However, they are not refundable.
You can decide what happens when your credits run out. You can choose to have your account limited to the daily free tier (2,500 lookups) or switch to the regular pay-as-you-go billing after your credits run out. You can set a usage limit on the dashboard.
If you have any usage to date for the month when you purchase credits, the credits will be immediately applied to your running balance.