How to set monetization limits

[#1]

Hi,

We have configured some products in our proxy. We are trying to configure the following rate plan for our users with the following requirement:

We enforce limits based on a custom attribute called "custom-unit", which is calculated in the proxy layer for every API call made. This is already done.

Two rate plans under monetization.

1. A Paid Rate plan: With 150K "custom-units" per month (monetized using rate plan with custom attribute) but no restrictions. Even If the user exceeds the limit, we want to let the use to make use of the API. The extra calls made will be charged as overages in next month, based the volume based pricing in the rate plan.
2. A Free Rate plan: With 10K "custom-units" per month (monetized using rate plan with custom attribute). Once the user crosses the limit, access to the API must be stopped. This is a monthly plan, so the usage is reset at the beginning of each month.

We have successfully set the rate plan charges and we're able to see them accordingly in our revenue reports. We need help to understand:

1. How to set an overall usage limit every month for the free plan.
2. How to ensure there is no upper limit on the paid plan for usage.

Can you pls help how to achieve this?

Thanks,
Jeevan