Many shops/dealerships might not like third-party extended warranties. Issues with getting paid for the work they did, the prices they charged, or justification in what they did and why. Some extended warranty companies even like to supply the needed parts themselves, many times from used parts vendors (as LKQ, which is an old insurance company designation for used parts "like kind quality" as what's on the car).
End result, is that YOU pay for the repairs, then YOU submit a claim for reimbursement from the warranty company. YOU paid them and YOU are left holding the bag for the repair cost if THEY don't pay up . . . at possibly a lesser amount than you paid for the repair. THEY have a good sales pitch for "One transmission replacement can cost more than the price of this warranty", by observation. Oh, and there can be a deductible, too!
What kind of issues are you concerned about? Just curious.
CBODY67