With eCart you can add in the functionality to add these items to the cart regardless of what price you want to pay for them. You can also generate a checkout that does not process the transaction using the checkout wizard.
As for some of the specifics that you have mentioned like the pricing based on the values entered into the fields and managing your products like adding or deleting the ones that you see on a page are not features of eCart. You can use eCart with a database driven site where the items are held in your db, but eCart does not have any management built in for managing your products.
You could create an admin area for managing your products with DataAssist, this is the exact type of thing it was meant for. If you have any further questions or would like to more specific details about any part of this please let us know.
