To prevent overlimit charges, you can reserve more traffic by changing your traffic limit to the bandwidth level you are expecting to have. For that you can impose a mimimum idle time between requests that is Idle throttle policy.
When the miminum is not reached, the request incurs a calculated delay penalty or is refused. First, whenever the elapsed time exceeds the period length, then the counters are reset. Second, if the idle time between requests exceeds the minimum, then the the request proceeds without delay. Otherwise the request is delayed between one and ThrottleMaxDelay seconds. If the delay would exceed ThrottleMaxDelay, then the request is refused entirely to avoid occupying servers unnecessarily. The delay is computed as the policy minimum less the idle time between requests.