As PE works now, you select a client’s invoice delivery method at the client level. Something to consider would be to add this feature to the Billing Wizard which would allow the creator of the invoice to select the invoice delivery method at the time the invoice is created.
This would come in handy for clients who have more than one service where some invoices would be created to be printed/mailed (i.e. invoices mailed with tax returns) and some invoices would be emailed (i.e. FMS monthly invoices or Assurance interim billings).