If you start a free trial, the free trial begins before you are first charged, so there are no payments to be refunded.
You can have 14 days of trial for any premium plans. You will not be charged if you cancel your subscription before the trial ends. So, it's totally risk-free.
For example, if you used 5 trial days for the Basic plan of $14.95. Next time if you try the Advanced plan of $24.45, you will have 3 trial days (because the total trial days is 7). To conclude, no matter how many times you try any premium plan, the total trial period is still 7. In case you already used up all the 7 days in one time, the next time we save 1 day for you.
To keep track of how many days you have left, you can see the message at the bottom of any page.