Does daily transportation of specific individuals for a monthly fee, after I swore to a police officer that I would not do that again (and my intention at the time was not to pick up passengers from the street), constitute breaking an oath, and can an expiation for an oath be performed to absolve oneself?

If you intended, when you swore an oath, not to pick up people from the street, then you do not break your oath if you pick them up from their homes. This is because the intention of the one who swears an oath specifies the oath, and scholars have agreed that the intention should be referred to if the wording allows for it and does not contradict the apparent meaning. Consequently, you are not required to offer expiation for your previous oath if you picked people up from their homes and not from the street, which you intended not to pick up from. This is with the understanding that obedience to the state is obligatory if it prohibits such actions for legitimate and considered interests.