We have always tipped ten dollars both ways. The ride is $80 I believe, divide that in half and then figure a 15-20% tip which is customary for services, add a little extra for the stop, and ten dollars comes up as a good round number. The grocery stop is something that they offer that is included in their services. Everyone I know that uses TTC has used the grocery stop at least once. The only time we didn't is when we ended up getting to town at 2AM!! Going to a store at that hour with tired children (and adults) wouldn't have been fun!
We tip the same for the ride back since the luggage is always heavier from all the extra's we are bringing home.