Informal bribe? It does sounds tricky.
If you have to give a tip to someone with a forceful smile on your face and the other party feeling entitled to it, done openly without any fear of being punished, that is more like a formal bribe 😅.
Truly, this act is more like a culture among uniformed men, it is the reality of the world we live in now. It is either we flow with the tides or get reproached for not falling in line.