[dsvzooiy] Reimbursing monthly transit pass

Begin the month paying bus or subway fares per ride.  At any point in the month, if you should decide to buy a monthly pass, it should be reduced in price by the amount you've already spent on single-ride fares.  Essentially, already spent single-ride fares can be applied toward the monthly pass.

This would work for the Boston MBTA Charlie Cards, where the cards can hold value for single-ride fares or can hold monthly passes, and the same card is used for both, making it possible to know which single-ride fares should be applied toward paying for a monthly pass.

This scheme removes from the regular rider the financial risk of whether a monthly pass will be "worth it" this month.  No free lunch, so it increases risk on the transit authority, but it can average out the variability over millions of passengers.