Is honesty alone worth $353?

I took Tina (my aging Hyundai Tiburon) in today to get some scheduled maintenance and a state inspection.  While getting this done, they informed me that I needed two new tires.  I was expecting this, so I assented.

After the maintenance was finished I was surprised to receive a bill of only $132.83.  While paying I questioned the cashier about this, remarking that this seemed really inexpensive for service that included changing two tires.  She pointed out that I had been charged for the labor for the tire change, but not for the tires themselves.  She didn't know why this was, but wasn't about to try to get it corrected.  But instead of just walking out the door at that point, I instead asked my regular service advisor, Craig, about it.  He realized the mistake, and called the service advisor who had handled my paperwork to the service desk to make the correction.  While this was going on, I mostly just stood there silently.  In the end, I was charged an extra $353 for the tires - the full difference of what I would have paid had the bill been done correctly.  The service advisor also thanked me a couple of times for pointing out the error.

So...  Should I feel better about myself because of this?  After all, I did basically volunteer to pay 350 bucks that I could have spent in other very satisfying ways.  (It may just be because I'm tired right now, but...) I'm not sure if I could come up with a Utilitarian justification of why I would have been obligated to report the mistake - I don't think the dealership would have missed that relatively insignificant amount of money very much.  Are my practical moral intuitions more Kantian (or perhaps something else non-Utilitarian) than I thought?  Maybe I'll ponder these questions sometime when my head is in a clearer state.