Variable invoices without line totals and only grand total

To add to the above, this is my enquiry from the 1st January
Ok so here’s the challenge
I want to quote and invoice all the items as a single price and still have control over the inventory. the reason is that we do not want to show the customer how we arrive at the total price

Let me explain how the invoices will work…
Single or multiple smoke detectors
or
Single or multiple smoke detectors with remote controller
or
Single or multiple smoke detectors with remote controller and one plugged (capped) mains cable
or
Single or multiple smoke detectors with remote controller and two plugged (capped) mains cable

Is there a way that I can hide the line totals and only show the whole invoice total on the customers copy?
To give an example, the following invoice and quote when printed will show $390

4 smoke detectors $320
1 remote control $ 20
2 plugged mains cable $ 50
TOTAL $390
As you can see the detectors and remotes are inventory and plugged cables are service items
Hope you can help as I am sure I am missing something that your experience can provide
Happy New Year
kind regards