Can I buy or top up credits as a one-off?

Yes. You would need to purchase a subscription and cancel or downgrade it before the next billing cycle.

Buying credits

To buy credits as a one-off, go to My Subscription > Change plan, then purchase a monthly subscription with the number of credits desired. For example, if you would like to buy 200 credits as a one-off, subscribe to the Pro subscription with 200 credits. As long as you  cancel it  within the same month, there will be no recurring charges, and you will have a month from your day of purchase to spend the credits purchased.

Top up credits as a subscriber

To buy extra credits as a one-off, upgrade to a monthly plan with more credits, then downgrade within the same month to avoid extra charges. When you upgrade, you will be given the full monthly credit allowance of the new higher plan, and receive a discount corresponding to any unused credits in your old lower plan. You will then have a fresh month to spend these extra credits.

For example, you purchased a monthly Pro subscription with 200 credits on the January 1st. On January 10th, you have used 100 credits, have 100 credits left but need 300 extra credits for another project you started. You then proceed to upgrading your subscription to the monthly Pro plan with 400 credits, and pays just $86.25 instead of $115 because of the 100 unused credits in your account. In the end, you end up with 400 credits valid until February 10th.

See  here for more information on plan upgrades and downgrades .