Gas pipeline contracts can be very complex if you get beyond interstate pipelines. If you are gathering wellhead gas or even just picking up rich gas at central gathering points, the company gathering the gas in many cases is also processing the gas, fractionating the liquids, marketing the gas and liquids, etc. Maybe the producer gets a guaranteed fixed liquid recovery or maybe they take actual recoveries or % of proceeds. The producer might have a right to reject ethane which changes the contractual liquid recovery structure. In other cases it's just a gathering fee or maybe a gathering and processing fee with volume redelivered to some other location.
Interstate deals are generally a lot less complex. There are accounting systems that handle all of the complexities.