I work as a delivery clerk for a pizza restaurant/store. Delivery clerks (drivers) provide/use their own vehicle to make the deliveries. At the end of the night the delivery clerk (driver) receives a small commission (payment) for the use of the vehicle used to make the deliveries done that day/night. The amount of the commission received is equal to approximately $.27+/- (cents) per mile. Needless to say, that commission is lower than what the IRS gives credit when a person uses their own vehicle for work purposes. My questions are:

1- Am I allowed to claim in my income tax return form the difference of the mileage that I am entitled to receive (assuming that the IRS still gives a credit of $.58 per mile)?

2- For 2010, what is the amount of money per mile that the IRS allows to claim, and is 20,000 miles still the limit?

1 - Yes, on form 2106 or 2106EZ. You enter the difference. But it's .50/mile, not .58, for 2010.
Keep in mind that you must have a log book showing actual dates and mileage (usually odometer numbers) in case of audit, and you cannot include miles gone for personal use. You can't just enter how many miles you put on your car in 2010.
2 - I don't remember a mileage limit? Where did you get 20,000?