Seems Track-it-back is the most lucrative for the highest mileage count? There are only so many "freebies" of thousands of bonus miles for a few dollars that it then comes down to Track-it-Back at 40 miles per $1 or FTD at 20 miles per $1 - correct?
FTD effectively becomes 50 miles per $1 - 2 cpm not so good. Track-it-back becomes 100 miles per $1, or 1 cpm. Cheaper than then buy miles promo. Not taking into account the other 4 shopping purchases you also must make, of course.