Well, it's 7.50, but I take your point. The short answer is that they have your credit card and the contract likely says they can add charges that you incur that you should pay, if they come back to the rental company.
What is the rental fee? Is it D$/day for every day you have the car, or only when you use the transponder?
I'm really unsure why there should be a fee at all. There's very little paperwork (heh, even if it's electronic) involved for the rental company
- the toll people ding the company, the company dings you when you return the car.
If you knew where the transponder was, you could faraday cage it, and then . . . what? I may not have thought this through.