You do'nt always get the same driver, so it is good to tip both ways. When we have a towncar and it's just the two of us, I tip $8-10 each way, depending on if we make a grocery stop or not.
On the trip I just returned from, there were seven in a van, including two young kids. We made a lONG grocery stop (I think it was probably over 35 minutes, as my sister sent my B-I-L in to the store, and 2. didn't give him a list UGH!!!) I tipped him $20 on the way to WDW, then an additional $10 on the way back, as it was just pandemonium getting us all in the van as we were leaving.
We had Humberto, I am going back in a week, and have already called to request him again, he was wonderful.
Anne