How do I invoice monthly guard hours?

Short answer

From the Pro plan, for a calendar month, 4guards computes the billing basis per contract: contracted hours compared with hours worked.
It applies the hourly rate from the contract to the hours worked (shift or site rates are not used) and adds overtime and the night premium where they apply.
It freezes the basis in a run (draft, confirmed, exported), so the figures stop changing, and exports it as a semicolon-separated CSV with 15 columns.
With the exported basis, you issue the invoice in 4invoices.net, where the series, number and fiscal document are set; 4guards does not issue invoices.
Collection and its reconciliation are handled in 4invoices.net or in your own billing system, not in 4guards.

Law 227/2015 on the Fiscal Code — the rules for issuing and the mandatory content of an invoice for services rendered.
Law 333/2003 on the protection of objectives — the billed rate must match the services actually delivered under the contract and security plan.

Example: a site has 2 contracted posts at an hourly rate. At month-end, 4guards computes the billing basis: 720 hours worked multiplied by the hourly rate in the contract, plus overtime and the night premium, if any. The basis is frozen in a run and exported as CSV, and the invoice is then issued in 4invoices.net.

Billing is based on the planned roster, not on hours actually worked — discrepancies only surface at reconciliation.
The rate is recalculated manually every month, with a risk of error applying contract terms.
The billing basis is not kept frozen per period, so the figures change after the invoice is issued.

A billing basis, from the Pro plan, per calendar month and contract: contracted hours against hours worked, multiplied by the hourly rate in the contract, plus overtime and the night premium. The billing cycle, notice and renewal fields in the contract are informational, and a contract past its end date drops out of the calculation.
A frozen run of the basis (draft, confirmed, exported) and a CSV export, so the starting document for the invoice stops changing.
The invoice is issued in 4invoices.net, which keeps the series, number and fiscal document. 4guards does not issue invoices and does not transfer data automatically: you use the exported basis as your starting point.