Credit and sanction collection vary internationally.
Please make sure that the few years ago don't exceed the 5 years credit collection limit, after which the fine is void.
At the present time a bus ticket or traffic sanction is not a valid refusal reason for the EU/Schengen area.
International traffic* sanctions can be enforced only by means of credit collection bureaus. The bus company is required to give the documentation to a credit collector that operates in your country, but it is also important that the two countries have bi-lateral agreements to enforce credit collection and guarantee legal validity to claims.
If there are no bilateral agreements, you can safely trash any collection letter as this won't be ever enforceable on your local court.
I don't have detailed info but in general in Europe if there is no law provision to detain you in such cases of fare evasion. Even if you visit Europe/Italy again no one will be legally allowed to say "Sir, you must follow the police now and can't leave until you have paid the fine".
This because European laws (namely, local laws in the context of European countries) strongly distinguish between civil violations and criminal violations, unlike for example the US system in which this could be likely considered misdemeanor and worth an arrest.
*Note: I wanted to improperly use the English word
traffic to include ticket evasion into the scope. As a non-native English speaker, I don't exactly know if the scope of
traffic violation is limited to driving motor vehicles or extends to public transport