Auto Send E-mail or Fax

I want to be able to "idiot-proof" our invoice remittance. Right now we give the customer the option to have their correspondance via E-mail Fax or Postal Mail. This is tracked with 3 custom checkbox fields on the customer record, one for each type. I have these fields sourced into the invoice screen so the CSR know if they need to e-mail, fax, or print and mail the invoice. Right now we rely on the CSR to look at these fields, and check the "To Be E-mailed", "To Be Faxed", "To Be Printed" whichever is relevant. After auditing the invoices,