The cards are stored-value - meaning that the card actually stores how much money it is worth. For cost saving and legacy reasons, most Clipper readers are offline and simply read and update the card value.<p>When you add value online, there is no way for the service to write to your card, so instead Clipper terminals throughout the system are informed of the value add and apply it next time they interact with your card. Online terminals, e.g. most vending machines, frequently synchronize with the online service and can transfer value added online almost immediately. Offline readers like those on buses, though, obtain a batched list of online value adds from time to time, often in the bus yard overnight via short range wireless networking - but in some areas this may happen less frequently than once a day. For Muni, I believe it should always happen overnight.<p>When you add value at Walgreen&#x27;s, they set your card on a terminal that updates it directly.<p>This limitation is common to all stored value systems with offline terminals, including all Cubic transit card systems such as Clipper, Oyster, SmarTrip, etc. It&#x27;s a compromise involved in rolling such a system out to a vehicle network without the expense of long range radio networking in every terminal - these systems were designed and implemented well before this was practical.