As a UAL guy, our contract states the following:

5-F-1-h: "The Company may offer Add Pay to a Pilot to waive contractual duty limitations, or to extend the flight duty period limits per allowable FAR limits when Pilot concurrence is required, provided there is only a single flight in the Duty Period and the offer is made and accepted before takeoff of that flight. If the Company requests and the Pilot agrees to waive up to seventy-five (75) minutes, he shall receive two and one-half (2.5) hours of Add Pay. If the Company requests and the Pilot agrees to waive more than seventy-five (75) minutes, he shall receive five (5) hours of Add Pay."

Not sure if this answers your question.