You can top up your credits manually on Silverstreet by logging in to your Silverstreet Dashboard (login link here) and then following the steps below;

On your Dashboard, go to More - Financial - Buy Credits

Here you can pay by card or Paypal or even set up automatic billing. When your credit balance is low Silverstreet will automatically charge your card to top up your credits. You can stop this at any time by deactivating it in the Financial centre.