Testator pre-approving professional executor fees

Has anyone here come across an estate in which the testator has signed a document agreeing to the level of professional executor fees which will apply after their death?

It has been drafted such that the testator agrees to the costs which apply at the time the firm is acting. However I wonder at the use of such a document as a shield to any challenge to their professional fees in a dispute.

Assuming the STEP Provisions have been incorporated, they can charge their standard fees but fees are always open to challenge?