If the rate quote shows unlimited mileage with that contract ID, then you should get unlimited mileage. Print out the rate and bring it with you just in case you're charged more.

However, in my experience the Big 10 codes (like 5000491) always come with the per-mile charge for one-ways. If you're trying to avoid the underage fee, try 5760339 and 5125601, which only add an extra $10 a day age-discrimination charge (any underage fee is too much, but $10 is better than $25)!