Once your payment is complete, WebCal.Guru knows about almost immediately. But your calendar application does not.
It is not possible for WebCal.Guru to tell your application that the calendar has updated. The server has to wait for the application to ask for new data.
The best option is simply to wait for your calendar application to refresh the content. Usually this takes up to a day.
For some applications it is possible to manually refresh the content. To see how this is done, please take a look at How do I force my calendar application to refresh a calendar subscription?