I know if a US seller uses it, the package, no matter where the parcel is coming from in the US, goes all the way to Kentucky to be checked and if needed opened and repackaged, so it is cheaper to send using the courier that eBay have designated. In all you will find that everyone that uses the service charges more for P&P, and the package will take twice as long to reach it's destination.
I know this because I ordered a record from the states, and when it took so long to arrive I did a check with the tracking number and found that it was at that time in Kentucky. I made enquiries with eBay and they told me the way it works. In all, instead of the usual week to 10 day to get a record from the states, it took over 3 weeks.