I don't know whether adding a car hire to an existing flight booking switches it from a flight only (not covered by ATOL) to a package (covered by ATOL) but there are benefits from booking an AVIS rental through BA that you don't get when booking direct with AVIS... extra driver free, extra AVIOS, sometimes extra days free etc. I have found though that when booking a car with AVIS through BA I don't get my Preferred number passed through to AVIS even though I put it on the booking so when I turn up I am not considered preferred.