I've had this happen a few times with Avis vehicles... and strangely enough it was on those annoying 'connected' cars where it reports back the (incorrect) fuel level. Its supposed to report the mileage also with that 'connected' feature but I've had some where the starting mileage was almost 0 so the rental receipt showed I drover something over 20k miles in 2 days. I questioned it at the return, but the rental was also unlimited mileage so didn't have to pay any extra for their mistake.

I'm wondering where you rented that you had .30/mi on the agreement? Most locations in the US are unlimited miles.

If they don't fix it within another day or two I would contact Amex and request they put a hold on the charge due to the obvious error.