in employee benefits cannot use that benefit to pay for rail travel or a day pass. Discounted (half-fare) cards have different rules for SmarTrip and CharmCard. SmarTrip for seniors and the disabled may only be purchased in person from a Metro agent or authorized sales office, and the person must show ID and for a disabled rider, proof of disability (either WMATA Disabled ID or Medicare card.) CharmCard for seniors and disabled are only available from the MTA reduced fare office at 6 St. Paul Street in downtown Baltimore, with similar requirements (proof of age or disability). Also, while WMATA charges a $2 fee plus the amount to load in value for senior/disabled SmarTrip card, MTA merely requires at least $2 in value to be initially loaded onto a senior/disabled CharmCard; MTA waives any fee for the card. "Exit fare" machines (used with paper farecards) previously did not accept SmarTrip, and riders with insufficient value to pay their fare were allowed to exit the system with any negative balance. This negative balance must, however, be paid before the card may be used again to enter the system. In addition, the card must contain sufficient value to pay the full fee in order to exit a Metro parking lot. Since SmarTrip owners were allowed to exit the system with an unlimited negative balance, the Metro Board had previously rejected proposals to reduce the price of SmarTrip cards from $5 down to $2.50. Starting September 1, 2012, Metro began offering $3 rebates to customers who registered their cards online after purchase. The cards still cost $5, but a $3 credit was refunded to the card five days after first use. Also starting September 1, riders have been required to have a minimum balance of $1.20 on their SmarTrip cards (35¢ for half-fare senior/disabled cards) in order to enter the Metrorail system, which reduced the possible negative balance upon exit. Effective October 1, 2013, the price of the card was reduced to $2, the rebate program was discontinued, and the maximum permitted negative balance upon exit was set at $1.50, with riders who would exceed this being required to use the Exitfare machines to add value to their cards. SmarTrip cards comply with the ISO/IEC 14443 Type B standard. A microchip contained within the card stores its value, as well as the rider's most recent entry and exit points, and a unique identifier.