Charge for tire replacement

We rented a car from Calgary airport and noticed soon thereafter that one tire was slowly losing pressure. After putting air in the tire and noting further gradual loss of pressure (which triggered the warning light) we notified Thrifty and they offered us another car. After the trip we then received a bill for $250 to replace the tire. We feel that the tire (which evidently had a nail in it) was leaking slowly when we received the car. What is our best action? Neither our insurance company nor our credit card company recognize this as a legitimate claim, and they will not cover the damage.