GNV TUNISIE D'ALESSANDRO AGENT

I bought a ticket for 4 people with GNV Tunisia through a Tunisian agency (North South Travels). I contacted GNV France to replace a passenger by another (service noted in their terms and billed at € 25), it is explained to me that GNV France can not perform this operation and I must go to GNV Tunisia. I contact them by phone and then everything changes, the reception is cold and he explains to me maliciously that the replacement of passengers is prohibited. I recontact GNV France and he explains that GNV Tunisia can of course modify the tickets. This time I go to GNV Tunisia at the port of La Goulette, I tell my problem to a man at the port, he asked me to wait and he went to see a person who works at GNV, he comes back telling me that the person agreed to change the ticket but you have to give him a bribe (rachwa رشوة) I ask the cost of his bribe he answers me 500dt. And yes the person wants to illegally 500dt to do an operation that does not even cost 25 €. My claim will not stop here, I assure you.