The MSC will pay the least amount that they have to in order to get the job done.
If Shopper A lives three miles from the location and requires a $6 bonus (PAD) and Shopper B lives 30 miles from the location and asks for a $9 bonus, Shopper A is going to get the job, even though the MSC is paying Shopper A $1/mile roundtrip and the MSC would only be paying Shopper B $0.15/mile roundtrip.
Unless, of course, Shopper C lives 50 miles away but is making a trip to the area for other work anyway and requests only a $2 bonus. Then Shopper C gets the job.
Unless, or course, Shopper D lives 75 miles away and is visiting family next door to the shop location and picks it up for base price while the the other three shoppers' PAD requests are pending.
Mileage has virtually nothing to do with it.
Ask for what you need in order to have it be worthwhile for you.