By default,  on invoices,  in client address block,  client's first and last name are used as "Attn:".  If a different attention to is needed for different cases for the same client,  please click on the Cases tab,  select the case and then click on "Other Info" tab on the right side.   In the field name please enter ATTENTION,  in the value field enter any string (such as a name or multiple names) and save.  Invoices for this case will now show the entered string as "Attn:" in client's address block.