I just did a 6 day rental and was a little shocked at my $4.50 "Frequent Travel Partner Fee" for United...which would have net me 500 miles.

Out of curiousity, I switched to my AA number, which would net me 300 miles....and the fee dropped to $0.37 for the rental.

So, just a warning to test out different FF programs because there is some funny programming going on there....