Two small things, both of which happen at the door, and neither of which is mentioned when you book.
1. Every guest's ID is registered — including yours
Every accommodation provider in Italy, from a large hotel to a single rented apartment, is legally required to record an identity document for all guests and report the stay to the police authorities. This is a public security obligation on the property, not a formality it can waive.
What that means for you
Bring the document itself. Prepayment does not replace it, and a booking reference is not identification.
It applies to everyone in the room, including children and including the people who are not the one who booked.
A self check-in still needs it. Apartment hosts usually ask for photographs of the documents before arrival, for the same reason. If a host asks for this, it is normal and legally required — not a scam.
Source: article 109 of the Italian consolidated public security act (TULPS), on the reporting of guests by accommodation providers.
2. The tourist tax is not in the price you paid
Most Italian cities of any size levy a tourist tax (imposta di soggiorno), charged per person per night. It is set by each municipality, and Italian law makes the accommodation provider responsible for collecting it from the guest.
That is why it appears at the end: the online travel site sold you the room, but the tax belongs to the town, and the property collects it on arrival or departure.
What to expect
It is charged separately from your booking, even when the booking says paid in full.
The amount varies by town and by category of accommodation, and national law sets it in proportion to the price up to a ceiling per night, with some cities operating under special arrangements.
Many places cap it at a number of consecutive nights, so a long stay is not taxed for its whole length. The rule is municipal, so ask the property rather than assuming.
For a family across a two-week trip with several stops, it adds up to a real number. Budget for it rather than being surprised by it five times.
Source: article 4 of Italian Legislative Decree 23/2011, which allows municipalities to introduce the tax and makes the accommodation provider liable for payment with a right of recovery from the guest; individual municipal regulations set the actual amounts.