I find it very interesting that IRS allows businesses to deduct 57.5 cents per mile while if I drive for medical care I can deduct 17 cents per mile and if I am helping out in my community as a volunteer, only 14 cents per mile. Although this forum stays away from politics, I do find it interesting what the government thinks of volunteer efforts.
When I am looking at distant shops I refer to money for 'mileage' more as money for my extra time in getting there. I already can deduct the very generous IRS estimation of what it thinks it costs a business to operate a vehicle. So lets look at a 50 mile round trip . . .
In my area, if I go North or South I generally can take the interstate and drive time to and from the interstate plus interstate miles will be total drive time of an hour and a quarter to an hour and a half. I would likely ask for $15-$25 extra. If I go East I don't have interstates but do have good roads with low to moderate congestion. Total drive time would likely be the same so extra pay would be the same. If I go West, because of traffic congestion, my trip could easily require 2 to 3 hours, depending on time of day required, so my required extra pay would be $25-$40 plus any tolls I might incur. Obviously to my West is the bulk of my town and I rarely get bonus shops going West because there are many shoppers who live in those areas who will happily do the work with no bonuses.
Now lets look at how you handle 'mileage'. You cannot 'double dip', so if a company 'reimburses' you 35 cents per mile, you can't claim the payment as a tax free reimbursement AND take your 57.5 cents per mile IRS deduction. I usually treat the 'reimbursement' for mileage as a taxable bonus so that I can take the IRS mileage, and I usually try to negotiate the extra payment as a bonus in the first place.