I have lost some data and configuration after my trial has expired. I think you should only power off the machine and lock it until user decides to pay.
at least, send a reminder/notification before deleting it
Unfortunately due to the high number of trial users that we receive we would incur a very large storage cost to store each of the trial users droplets even for a 24 hour period of time.
The trial is really intended as that, just a trial, to see the control panel, and launch and play with a virtual server, if you are beginning to build production configs and install applications we would strongly recommend that you upgrade your account to paid so that the virtual server doesn't expire.
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