I am using Turbo Tax for Small Business. I know my mileage usage for my business, but not my TOTAL mileage. Should I just estimate?

You can estimate the total mileage because you will need it in order to claim the business mileage. 

Be aware though that in the event that your return is audited for some reason and the mileage is examined, the IRS will expect some documentation justifying the business miles and the total miles.  Check you oil change or auto repair records near the beginning and the end of the year.  They generally would include your odometer readings. 

Thank you! Yes, I was planning on using my oil change info to estimate. My business mileage is to and from the post office. I have postage receipts that verify when my packages shipped, which explains the trips to the post office on those days.