Every time you submit the state of the files you want to watch, it counts as a single reading. This reading can include multiple files, and additional data.
You can use all the features for the 15 days time period. If you cancel before the trial ends, you'll not be charged.
If you upgrade your plan, the change will take effect immediately, and the cost of your current plan will be applied towards the new plans cost. If you downgrade, you'll stay on your current plan until your current billing period expires.
You can cancel at any time. You'll be able to continue to use the service until the paid billing period expires.