A tip is voluntary and given as an expression of appreciation. A bribe is paid or not depending upon your calculation that what's needed or wanted can be obtained without paying and without being abused in some fashion for not paying. A demand for payment from anyone other than an official is classified under law depending upon the presence or absence of a threat to harm, physical contact (assault), amount of payment in demand and more. A demand for payment by an official, accompanied by an inferred or specified negative consequence if payment is not made is classified depending upon the official's position, authority (power), responsibilities and more. In general, an official who uses his or her position to illegally obtain payments in any form is said to be corrupt.
The cruising environment
is characteristically coercive in nature. Whether we are paying "port fees", forced to hire agents we don't need, are confronted by 'requests' or 'demands for payment by anyone - particularly those in positions of authority like immigration or customs
officials - there is usually an accompanying threat, inferred or otherwise, of negative consequences if we don't acquiesce and pay. In the case of 'port fees', we have no choice under law but to pay them. But, universally, documents telling in precise terms what must be paid are supposed to be given to anyone requesting permission to enter the country. Payment 'requested' or 'demanded' by an official of government
over and above these amounts is illegal. Absent documentation
specifying what we are legally obliged to pay, no payment should be made. And, if you feel there's no choice but to pay, demand a receipt. It's a good idea to carry your own receipt book in the likely case an 'official' feigns misunderstanding. The receipt is your proof a payment was made and should be presented along with your complaint to the police. It's unlikely immigration superiors are any more honest than the official just paid. And, if you ultimately end up having to meet with an immigration superior at least they'll know you've already been to the police. Remarkable how their behaviour is likely to change from uncooperative to accomodative.
Whether you pay bribes or tips and visit places requiring you hire an agent is your choice. But your payments and visiting places where using an agent is mandatory affects others who follow in your path.
Regarding my experience(s) or lack thereof some have used to attack my credibility. I'll answer in saying my voyaging to different countries and exposure to the subject(s) comprising this discussion are more than adequate.