This has been brought up before but bears repeating.
You CAN reload (add value) to your CharlieCard on a bus or trolley. Drivers have confided to me that the T really does not want people to know they can do this as it may cause delays boarding a bus or trolley. However since retail sales locations are few and far between and not everyone goes through a subway station everyday it is an option that riders can use. The T says
on-board fareboxes to add stored-value can hold up
boarding lines and cause transit delays. Consider loading value at full-service
in-station fare vending machines or retail sales terminals prior to boarding.
1. Touch the white button at the top of the farebox to begin adding value to your CharlieCard.
2. Tap your CharlieCard to the black card target and follow the directions.
3. Insert money—fareboxes accept cash, coins and even old T-tokens—and press the white button again to 'accept' the transaction.
4. Tap your CharlieCard again, a second time, to register the value you just added and you're done.
Then you can tap your card again and it will deduct the CharlieCard fare for that vehicle.
IMPORTANT The amount you insert in the farebox will be the total added to the card. There is no way to provide change and the transaction must be in cash.
I would suggest that if you need to do this simply tell the driver that you will wait until everybody else pays their fare and then do it. That will make the driver very happy. You may run into a driver that will claim you can't do load a card on a bus/trolley. Just show him how it is done.