Not to take this too far off-topic, but it's worth mentioning that picking up a transponder yourself to avoid having to use the rental company's is generally pretty easy--and that avoids the whole scam entirely.

FWIW, I carry three toll passes with me:

-E-ZPass issued by Massachusetts (no monthly fee and no charge for the transponder device itself); it lives in one of the compartments in my backpack and works on all E-ZPass roads (Illinois to Maine). Free transponder with payment of a $25 starter balance
-NTTA Starter TollTag; the sticker version (without protective cover removed) lives in my wallet and works on all toll roads in Texas, Oklahoma, and Kansas. Free tag with payment of a $20 starter balance.
-SunPass; the sticker version (without protective cover removed) lives in my wallet and works on all toll roads from Florida to North Carolina. Initial cost was $4.99 plus a minimum $10 starter balance.

The sticker ones are supposed to be attached to the windshield to work, but I've found they work fine just holding them up against the windshield when you go under a toll gantry.

It's three accounts online to create, but it's easy enough to set them up for credit card replenishment and just forget about them (unless you need to change cards).

And no, none of these agencies care one iota whether your mailing/billing addresses are in their service area. Massachusetts happily mailed me my E-ZPass to Pennsylvania despite the PA Turnpike operating its own E-ZPass product, though if you happen to actually be in Massachusetts, you can of course grab one there.

(I used to have a Bay Area FasTrak transponder, but heaven knows where it is, and fortunately, I don't drive toll roads in California very often. The day when all these systems meet Congressionally-mandated interoperability will definitely be nice...)