HOV/Carpool users need the Switchable HOV Transponder to drive for free in the U.S. 36 Express Lane. The cost for the transponder itself is $15. To request this transponder, log in to (or create) your ExpressToll account here and visit the Add/Edit Vehicles tab. 

If you won’t be using the Express Lanes in HOV/Carpool mode, then a regular ExpressToll sticker tag is acceptable and is free to obtain through the ExpressToll website.