The terms used sometimes confuse the subject, as in "completely fair" vs. "fair" vs. "moral".
One fundamental problem is that if you objectively as a third person look at many transactions, you will see that one person gets more value than the other. Frequently!! You can conclude that most transactions are unfair.
What makes it fair, or just, or enforceable is the fact that there was an unforced agreement, a voluntary one.
The "agreement" is what makes it voluntary. Voluntary meaning "not tricked into it" or not threatened by the other party into it.
We are not born with the ability to make the best transaction all the time, we learn to make better and better ones. Some people have low self esteem and are consistently taken advantage of. In many of these cases, resentment builds and they will not transact anymore.
The example you bring up is more about what is workable or practical or a best practice for one of the transactors. To observe the other person and IF a long term relationship is desired to make sure that the other person is satisfied as to not create problems later on. It is not about fairness, it is dealing with your own rational self interest (personal ethics).
But what if a rational person does business with an irrational person. Does the rational person have to determine what the irrational person should get? In most cases, it can't be done.
You may say that 15 dollars is what they should get, but then I think it should be 50.3425 dollars per hour. Why? It feels right to me.
Here you are controlling the process of transacting as a third person, as an authoritarian. Don't they have a right to be free to transact? Without you refereeing it? Or perhaps regulating it?