Where does eBay get its so called estimated delivery dates?

I've just moved to a remote isolated community which is a 150km drive from the nearest town with normal mail delivery in/out. Mail only comes and goes Tuesday and Friday. I'm concerned that when I start selling again I may receive more than the average "slow to post" ratings or defects. I will of course detail my location in the listings but it does make it hard when buyers leave feedback and it asks if it arrived by a predetermined date.

My previous location was also isolated but not to the same extent and it used to tell me that parcels from interstate were 2-4 days delivery estimate when the reality was more like 7-8 days. Even the average non priority letter is 4 days now. I never left adverse feedback for delivery times as I knew it was my location that caused any delays but should I be concerned as a seller?