-Options can be passed as a hashref (recommended) or as a list of up to
-four values for templatename, agentnum, invoice_from and amount.
-
-I<template>, if specified, is the name of a suffix for alternate invoices.
-
-I<agentnum>, if specified, means that this invoice will only be sent for customers
-of the specified agent or agent(s). AGENTNUM can be a scalar agentnum (for a
-single agent) or an arrayref of agentnums.
-
-I<invoice_from>, if specified, overrides the default email invoice From: address.
-
-I<amount>, if specified, only sends the invoice if the total amount owed on this
-invoice and all older invoices is greater than the specified amount.