In the case of gasoline, the company in question will pledge $0.01 for every gallon of gasoline that you buy.
Now, I happen to know from prior education that profit margins on gasoline are razor-thin and it also seems a bit ungrateful to turn my nose up at someone offering 'free money', but the economics of saving a remotely substantial amount for college in this manner is pretty laughable.
Let's say the average person in the average major metropolis (like say, sprawling Houston) refills their 15 gallon tank once a week through normal driving. Through Upromise that level of consumption would result in:
$0.15 per week x 52 weeks per year = $7.80 per year
So if you enrolled in Upromise on the day your child was born, by the time he or she was getting ready to tour college campuses you'd have a staggering $140.40 socked away to help defray the costs of higher education (and that's also if you made sure to seek out the participating company's service stations each and every time the fuel gauge needle was overlapping the E).
Never mind eighteen years of inflation - even in today's dollars that might not buy a single textbook. (Maybe if both parents Upromised you'd get that cost-free copy of Intro to Psychology)
Granted, if a million people took advantage of the offer Gasoline Company X would be out $140,000,000 in philanthropic donations over an eighteen-year-period but given how measly the savings are at an individual level I imagine that most people take a quick glance at the offer and say "eh, not worth my time to fill out the online registration. no thanks"