Hallo…
I’ve got a job set to start on December 10, and I’m curious if it’s cool to cancel before I even get going. What’s the usual policy for this kind of thing?
Has anyone else been in a similar situation? I’d love to hear your stories or any tips you have!
Regards,
Employment is frequently “at-will,” which means that either the employer or the employee may end the job relationship at any moment, for any cause, and without prior notice1. This usually covers the time leading up to your start date.
It is advisable to let the recruiting manager know as soon as you have made up your mind. An email will do if a phone conversation isn’t feasible, as it is a more intimate and businesslike communication2. Communicate clearly, explain the circumstances, and offer your apologies for any inconvenience.
Check your contract for any provisions pertaining to termination prior to the commencement date if you have signed it. 
Certain contracts may contain provisions pertaining to notice periods or fines.
The hiring manager will probably value your candor and quick communication even if they are not thrilled with your choice.
By using this strategy, bridges are avoided and positive relationships are maintained.