Note, there are several issues here.
Lets say that it is a reimbursement only job. You drive 20 miles total to perform the shop. The MSC rejected the shop. You are entitled to deduct both the mileage and your actual expense up to but not more than the amount originally agreed to as a reimbursement.
Lets say that it is a fee of $15 with a reimbursement of up to $5. You drive 20 miles total to perform the shop. The MSC rejects the shop. You are entitled to deduct both mileage and your actual exense up to but not more than the amount originally agreed to as a reimbursement. You are not allowed to deduct the $15 fee you would have earned had the shop been accepted.
Lets say that it is a fee only shop of $15. You again drive a total of 20 miles. The MSC again rejects the shop. You are entitled to deduct mileage and that is all.
Now lets say that the MSC accepted the shop and then went into bankruptcy and never paid you a dime. You treat it exactly as you would treat a rejected shop as outlined above.