We also tip $10 each way. That's a total of $20 for an $80 service (before coupon). I think 25% is a very good tip (even if the trip includes a grocery stop). I expect good service for that amount. If I didn't get it, the tip would be much, much less. FWIW, 15% would be $12 or $6 each way.
We also tip separately, because we can never be sure that we will get the same driver both ways.