Cost for Checked Bags

Are you considering the high- speed train? You may arrive faster by train, taking in account early airport check-in times and delays. We just traveled from Lake Como to Naples in May, and it was the exact same amount of time to fly as to train. We chose train.
Safe travels!

Every airline has different fees

Right, every airline has different fees.
Just curious: Are you looking at flying into Milan Malpensa (which is approx. an hour outside of Milan) or Milan Linate?

Just to throw out one option, here's the baggage info for ITA:
Allowable hand baggage size/weight::
https://www.ita-airways.com/en_us/fly-ita/baggage/hand-baggage.html

Checked bags: size/weight and charges (domestic and specfic out-of-Italy destinations)
https://www.ita-airways.com/en_us/fly-ita/baggage/checked-baggage.html

Every airline has different fees

And the fee can vary depending on when/where you pay for the bags.

And the fee for the 3rd or 4th bag may be more than the 1st two.

The question as posed is too broad to produce meaningful answers.