A calculation field set to return text can do this:
If (isempty(checkboxfield), "lead or contact" ; "customer" )
However, as Mr Vodka advised in another thread, you may want to consider a more flexible, normalized design.
Specifically, two tables, Invoices and Line Items where each order generates a single invoice record plus one related line item record for each item ordered on that invoice. The individual line item records can be placed in a portal which is in turn located on an invoice layout.