> He quoted from Rav Zilberstein that when one enters a
> sherut in Israel (taxi with many passengers going to one
> place) one usually pays up front. ... Seems strange to
> me that paying any worker in advance is less of a mitzva
> than paying at the end becuase he is not legally required
> to pay in advance.

If I'm understanding this correctly, the problem is that by paying in 
advance, he never had any debt with which he might do the mitzva of 
paying on time.

However, it seems to me that there might be other mitzvos involved: 
By paying in advance, he does a great chesed to the driver, who can 
rest easy and not worry if this passenger might be among the few who 
try to avoid paying. If this driver has had bad experiences with such 
passengers in the past, paying in advance would be a Kiddush HaShem 
as well, I suppose.

Another factor could be the fear that if he doesn't pay the driver 
now, he might forget to pay him later, or for some reason lose the 
opportunity to pay him.

Whether these considerations (and possibly others) might outweigh the 
lost mitzva of paying on time, is another discussion.

Akiva Miller