The post-renew option is a setting which allows services to be renewed and invoiced at the end of a given period instead of in advance.
When adding a new service to a client in the Client Manager, set post-renew to 'yes' on the Billing Info step, and it will be billed at the end of its first renewal period.
If you wish to change an existing service to post-renew, click Edit on the Service Details page, set post-renew to 'yes', and change Renewal Date to the to the beginning of the period that post-renew billing should start.
If you are using the daily invoicing method, all post-renew services will be invoiced directly on the day that they renew at the end of a period. If you are using the monthly invoicing method they will still be included on the prescribed invoice send date, but will always invoice for the prior period.