For security reasons, the sum of tickets purchased in one month cannot be more than 516.46 euro.
If you want to make purchases beyond the overall monthly limit of 516.46 Euro, you need to register at the Trenitalia Help Desk.
Registered Customers can check the amount spent in a month on their own "Order Summary" page, accessible from the Customer Area.

At the moment you cannot request an invoice for tickets purchased online.