Credit Card Penalty Payment

I mailed a credit card payment at least three days in advance of it due date, but a late payment fee still appeared on my bill. The credit card company informed me that it does not matter when I send the payment, but when they get it. This leaves me at the mercy of the post office, and at the discretion of the truthfulness of the credit card company. Is this legal? If not, what recourse do I hav e ?