I know with my i-Pass the transponder is a refundable fee should I no longer need it and turn it in. I don't recall if the $20.00 Surpass was/is refundable if I no longer need it, but for all the times I seem to be traveling down to Florida lately it's well worth it if the $20 isn't refundable.
You do load your sun pass account online with I believe a minimum of $10 and you can set a limit so when your account goes below a certain amount a credit card is billed an additional $10 - $20 is put on the account. That way no cash is ever needed for the toll system, and should you decide you never need a surpass account you can have the remaining balance in your account sent back to you.