I'm not sure if this holds true equally among different airlines, but we had a recent frontier airline purchase of less than $60. It has been almost a week, not reimbursed.

The flight info appears on my credit card statements, which may explain why amex isn't reimbursing it. I'm pretty sure my mistake was not doing something to make sure the amount paid was different than the price of the ticket.

3 weeks later - confirmed, it did not work