There is an invoice (billing order) released for the trial subscription. Why did it happen?
Trial subscriptions are billed for resource overusage exactly the same way as the subscriptions on the ordinary periods - at the end of period resource overusage fees are calculated according to the resource rate prices.
For the trial period to be completely free, not only zero recurring prices for the subscription period are required, but also zero resource rate overuse fee. You can setup resource rate periods:
Home> Products> Service Plans> #ID > Resource Rates > resource > Resource Rate Periods
so that overuse fee on trial period would also be zero.
Subscriptions synchronization is required for changes to apply to already created trial subscriptions.