Sub items in invoices or quotes

For example. currently when create invoices/quotes we only able to add 1 item.
Isn’t possible to add a sub items inside ?
I create a product which name “Product 1”.
But inside “Product 1” have some subs item and price

Product 1
-Item 1
-Item 2
-Sub Item 1

I try to develop but it’s hard.

Could you add an idea where something like this would be useful?

[size=10]Last Update: 15.07.2015[/size]

When create “Invoice” or “Quotes” for customer
Because sometime user might not only add product per product
Probably Add a set where this set include all subs item.

Is very useful for a lot company .
Probably A check box when add a item where isn’t this a main item. if it is, then can select subs item to add in. So when show in PDF . it will be something like the example i provided above . Thank you

Can have a look on this pdf :

SO when generate, instead of inside description 1 by 1 . it should list all the price in the sub list if possible.