Why a contract needs an official mark first
A contract between two firms is a private paper. Nobody from the state signed it, so there is no seal for an office to check. That is the whole hurdle here. If you are asking how to apostille a commercial contract, the real first step is to have an Italian official certify the signatures on it. An apostille is one stamp a Hague country adds so another country takes the paper with no embassy step.
Prefettura or Procura
Plain business papers on the administrative side are stamped by the Prefettura, the prefecture, which is the state's local office in Milan. Sworn and notarial acts go to the Procura della Repubblica, which sits at the court. So the form your contract took decides the desk. You can only apostille a commercial contract in Italy when an Italian official signed or sealed it here.
Cost and timing
The apostille cost for a commercial contract in Milan is a €16 tax stamp per document, with some papers exempt. Add the fee the certifying official charges, which is set by them and not by the state. Waits move with the queue at the desk, so build slack into any closing date. Start early when a tender or a shipment hangs on the paper.
What the stamped contract is for
Firms send stamped deals abroad to enter a tender, to sign up an agent, to clear goods or to back a claim in a foreign court. A bank may want one before it funds a shipment. To apostille a commercial contract in Milan for a bid, work back from the closing day, because a late file is simply out.
Annexes, language and acceptance
Keep the annexes, the price list and the schedules bound in as one set, and have each page initialled if the other side asks for that. The legalisation of a commercial contract in Milan ends with the apostille when the reader's country is in the Hague group. Many desks then want the text in their own tongue as an asseverazione, a sworn translation that a court in Italy stamps. No language is fixed here, so let the reader name it.