def find_invoice(invoice_id)
      builder = Builder::XmlMarkup.new(:indent=>2)
      builder.instruct! :xml, :version=>"1.0", :encoding=>"UTF-8"
      request_xml = builder.select do |select|
        select.invoiceNumbers do |numbers|
          numbers.invoiceNumber invoice_id
        end
      end
      response = post_xml(request_xml, {:action => 'select', :type => 'invoice'})
      InvoiceParser.parse(response) rescue nil
    end
def find_invoice(invoice_id)
      builder = Builder::XmlMarkup.new(:indent=>2)
      builder.instruct! :xml, :version=>"1.0", :encoding=>"UTF-8"
      request_xml = builder.select do |select|
        select.invoiceNumbers do |numbers|
          numbers.invoiceNumber invoice_id
        end
      end
      response = post_xml(request_xml, {:action => 'select', :type => 'invoice'})
      InvoiceParser.parse(response) rescue nil
    end
Finds an invoice by invoice number