How to give money to someone without being awkward?

I mention anxiety in this post, but my question is regarding social skills, anxiety was just an inciting incident.

So, to summarize the situation: I have severe driving anxiety. I've been in therapy for it and I though it was getting better. Me and sone friends from med school agreed to carpool to our rotation, and I though i'd be able to handle it. Well, after the first day when someone else was driving, I saw the crazy traffic and got anxious and told them I wasnt sure id be able to drive without panicking. They were understanding and still agreed to let me ride with them. I offered to pay gas money but they told me not to worry about it.

I still want to pay them back. How do I go about giving them money without being weird about it? Maybe its a simple answer, but i lack the social skills.