If I were going to drive to deliver anthrax, I would not pay with a credit/debit car, I would have paid with cash.
Absolutely. However, it appears that he used a debit/credit card to fill his tank presumably in the vicinity of his home rather than along the theorized route. Had he filled his tank say 100 miles from home, you can be that would have been printed in bold flashing fonts.
By the time a person is 60 years old, they have fairly routine behaviour. An anomalous ATM withdrawal of cash to cover the trip would also show up as different from the normal pattern of withdrawal. No mention of such withdrawal by the Feds.