Make sure that the quote that you give them is for repairing the sills with NO FILLER. Insist on no filler, no bondo. I doubt that you can find someone who will do that. Explain to the shipper that the sills get so hot that the filler will separate within 1 year and they will have to pay for it a second time. I don't know that any of that is true; but, neither do they. And even if the hassle factor with them is too high; to me, it would be worth paying the difference and getting JonB's OEM sills.
Take photos, so that if the next prospective owner finds this thread, he knows exactly what was damaged, and that it was fixed correctly so that he has no reservations about buying the car.