In free plans, there is no overage allowance. Once the allocated free credits are spent, open environments are paused. You will need to wait until the next billing cycle to receive new credits or upgrade to a paid plan (which takes effect immediately).
Open environments consume the monthly credits granted by the subscribed plan. Once the monthly credit allowance is exhausted, any additional usage is considered "overage" and will be billed at €0.18 per credit at the end of the current billing period.
Credits are only billed if fully consumed.
If you use 0.85 credits, you are not charged. If you use 1.25 credits, you will be billed for one credit. However, any unbilled remainder is carried over indefinitely.