Thank you Bellini
I think i have found the problem.
Carriers, for some reason, can only be configured according to countries in prestashop backoffice.
For example, imagine therefore having the same transport costs across the whole of the USA, Italy or UK etc. It is impossible to have same transport costs when the customer is located a few kilometers from the store and when located at the island. Especially when regarding bulky items.
I read at somewhere that I can overcome this problem by creating many countries eg. Italy - main land-, Italy Islands, North Italy....
The problem however is that different ISO codes are used, which are not recognized by paypal.
Can therefore in prestashop back office the same iso code be used by more than one country, without an error window popping up?
Thank you