I'm an Italian tourist, and I bought things here in Japan. Is it possible to have a tax refund at the airport? In particular I am interested in Narita airport. In case it is possible to specify, what kind of commercial products are eligible? (E.g. electronics, clothes, food, etc.)
Short answer: no, you can't claim tax back at the airport, you claim it from the store at the time of purchase. Eligible goods are everything except cosmetics, food, alcohol, cigarettes, medicine, film and batteries.
Long answer: Typically you obtain this discount directly from the store where you purchased the item, at the time of purchase, if they offer that service.
For example, Yodobashi Camera in Akihabara offers the full 5% off if you show your passport. However, many stores, and almost all smaller stores, do not offer 'duty free' shopping. An example of this is Yodobashi Camera in Chiba City (east of Tokyo). As it's not a 'tourist destination' they don't offer tax-free shopping.
As a general rule - if it's a tourist area, and a big store, then you'll probably get the discount. Otherwise, it's unlikely.
Furthermore, you must spend a minimum of 10,001 JPY in one transaction.
Source: Japan National Tourism Organisation Sales Guide, personal experience