This code automatically creates an invoice at the start of the month and notifies the customer.

getpaid-automatically-create-invoice-each-month.php

add_action( 'getpaid_daily_maintenance', 'getpaid_generate_invoice_monthly_1611816820' );

function getpaid_generate_invoice_monthly_1611816820() {

    // Check if this is the first day of month.
    if ( date( 'j' ) === '1' ) {

		$invoice = new WPInv_Invoice();
		$invoice->set_user_id( $user_id ); // Replace user id with the user id.
		$invoice->add_item( $item_id ); // Replace item id with the item id.
		$invoice->set_due_date( strtotime( '+5 days', current_time( 'timestamp' ) ) ); // Set the due date five days from now.
		$invoice->save(); // Save the invoice and send a notification to the customer.

    }

}