Add to Shopping Cart (not order) in admin?

Has anyone come up with a way to to add to a customers shopping cart (not to their order). Here is why I want it....

We offer mail-in repair. The diagonis charge is a flat fee. And easy to setup as an item the customer can dump in to the cart themselves. However when we repair the item that they sent in the price could very a lot and may need additional parts.

I would like to add a item into their cart, through the admin back end, even a single item called "repair cost" and enter in the amount. And then instruct the customer to login to enter their credit card information and accept the terms and conditions.

I would like to add a item into their cart, through the admin back end, even a single item called "repair cost" and enter in the amount. And then instruct the customer to login
Consider creating a new product - repair cost - priced at $1.00
Communicate with the customer to buy that item with a XXX qty and checkout