Here is a video describing how TravelPass as a subscription model works.

When travelers are planning a trip, they need to research, book, pay, change and maybe even cancel the trip. A subscription model turns a shopping process into a booking process, removing the payment step, saving time, cost and increasing customer share. Travelers can chose between a prepaid number of trips, or an unlimited number of trips in a defined period, or a postpaid solution. With TravelPass, travelers can quickly book, change and cancel a trip without the need for time consuming and costly re-issues or refunds.