I would understand and even appreciate that as far as that practice was restricted to how commercial traffic was handled within itself. But whether the individual operators received any incentives for handling passenger messages or otherwise, IMO messages related to navigation should still receive priority over them under all circumstances. From what Julian and you have said, I assume that was what in the 'Rule Book' anyway.
I have always believed that that £5 was Cosmo's effort to help the crew in his boat and
not meant as a bribe; I have no doubt that one of the newspaper reporters somewhere got the idea that it would make a more sensational story if he/she described it as a bribe so that the crew of the underfilled Lifeboat #1 would not insist on going back to pick-up more people out of the water. The related scene in the film version of ANTR helped to foster that 'bribe' idea for later years, depicting Lucile Duff-Gordon as some sort of selfish b***h.
OK, but even if they were not paid a commission - ie an agreed fixed official sum - for private messages, would Marconi Company be in a position to make sure that their operators did not accept tips whilst on board a ship? In that regard, I would like someone to tell me how far "Out of Bounds" the wireless room was considered for passengers; even if it was, Bride and Phillips were not confined to their quarters and had to mingle to have their meals etc. So, IF people like Astor, Strauss, Hays, Widener, Thayer etc, who had a range of business interests, wanted to have a "quick word" with either of the two operators, IMO that would have been possible. I am not saying such things actually happened, but just considering the possibilities.
It is sometimes called "Gratuity" in the UK but "Tip" is also widely
accepted - both literally and materially
.