If you do not use any CDP, or if you use the AAA CDP, it says unlimited free miles. That may be cheaper than your corporate "discount," since it's a long distance one-way.

Does it say a mileage charge on the confirmation, or are you calculating what the mileage charge "would be" based on distance and increased price?

If the former, what CDPs are you using? Some corporate CDPs that charge mileage for one-ways do not do so within a certain distance, or a slightly higher base rate. Ex, roundtrip, $45/day, unlimited miles. For one-ways over 200 miles, $45/day plus 0.29/mi. For one-ways less than 200 miles, $65/day, unlimited miles. It seems a fixed corporate rate for shorter one-ways is being phased out.