Internal Item Only - not display publicly - ex. assembly

Some customers request assembly or other such customization

This is not a cookie-cut kind of thing that can be listed alongside an item as an option cuz there are lots of variables in doing the assembly or whatever

However, we still need to quote an assembly charge and include it somehow in the order.

So, would be most great if there could be an item that is for internal use only that would add to an order, send the adjusted amount out to the customer via email, and then allow the customer to pay for the customized order by clicking a payment link or something, maybe log back into their account.

This customization/assembly "item" should not be displayed to the public, just available for staff/admin to make an adjustment to an order and email the quote, get payment, etc.