As I haven't seen, much less am I familiar with, the inner workings of e-junkie shopping cart, my comments below are based upon my present setup in my HTML page.
I should reiterate as I think I may have confused you. In reply to the 'amount' hidden field, it's actually a price field to the right of a 'text' field where the customer enters/types in their first name or initials. To be clear, the price varies according to the length of the string (number of characters in the name) entered into the text field to determine the total price of the letters, priced at $2 each . The idea is let the customer see the actual price develop as they enter their name into the field. Granted, there are a multitude of other ways to accomplish this task, most of which are totally lame in that the customer is making a zillion selections while going back and forth between the shopping cart and the product page. Please understand that my primary goal is to make the shoppers experience a faster, easier and much more pleasant one than I am accustomed to seeing at the majority of other online stores. A good example or case in point is hipcandy.com Needless to say, ask anyone, my daughter and granddaughter included, and they will tell you that this is a complete total disaster to say the least.
Once again, while I am not familiar with e-junkie, as a programmer, it makes sense that I should be able to determine the cost of the letters for the shopping cart without having to use the calculated price field on my product page which is essentially for the customer only. Hence, this should really be no different than a line item with a fixed price meaning (depending upon how I can set this up in e-junkie), the shopping cart determines and shows the letters priced at 'x' amount of dollars based upon input inside the text field. If the customer should change his mind and decide to empty the cart, so be it, all goes back to 0.00. By not allowing the customer this option in order to satisfy my own needs is, IMO, a Big Mistake!
In fact, when you think about it, there should really be no need to begin with to disable the Buyers ability to edit Product in the shopping cart. If he/she decides to delete the letters (one item), he/she deletes all of them. I believe the confusion lays in the letters as they are not being added individually to the shopping cart in this scenario but rather as a group defining one price for one item in the cart as opposed to purchasing the letters individually. For all practical purposes, the description in the cart could be 'Letters', 'John'. Trust me, from what I've seen, most web designers/developers would have 10 separate drop down boxes (for the 10 maximum letters I allow in the text field) making a total of 10 separately priced items together with 10 trips to/fro from the shopping cart. I don't think so!
Last but not least, you forgot to answer my question about 'FatCart'. Is it capable of doing any of the tasks I have described above?