I topped off my last Charlie Ticket today. The Charlie Ticket is the little paper ticket that you can use in Boston to pay for the trains. I've got a new Charlie Card now, which is plastic.
My last Charlie Ticket had $1.50 on it. I topped it off to $1.70, but the ticket was rejected because $1.70 is the per ride fare if you use a Charlie Card, not a Charlie Ticket, so I went back and topped off the ticket again from $1.70 to $2.00.
I did these transactions with a credit card too!
It'd be much easier if the MBTA standardized it's fare structure.