If your sister lives in California, then she is likely familiar with electronic tolls via FasTrak. Same basic premise to be considered here; if not using a personal transponder options are to pay the higher cash rate, wait in the cash lanes to pay (note that many states are going to cashless tolling) or pay the higher PlatePass fees. It's a personal decision if these costs are preferable to the effort to obtain and maintain a personal toll account for an area in which one does not live.