When you transition from a free trial user to a paid subscriber, the trial period ends and any billable usage will be charged according to Wasabi's standard pricing policy. This applies regardless of when you change from trial to paid in the 30 day period.
For example, if you sign up for a free trial on January 1st and after doing some initial testing, you decide to become a paid subscriber on January 3rd, the free trial period ends and storage and egress usage charges commence.
If you have any active storage in your account at the time you transition from free trial to paid subscriber, the usage charges for this storage will commence on the day you become a paid subscriber. In addition, if you have any deleted storage or egress usage during the trial period, those usage metrics are restarted from zero once you become a paid subscriber. Lastly, all active storage and storage buckets that you have in your trial account at the time you transition to a paid subscriber will be retained.
In summary, you will never be charged for any type of usage during the trial period.