We've got several products that are fixed price, but we'd also like to add the capability for users to make online bill payments or arbitrary values.  Essentially we'd like to be able to turn on "user entered values" only for specific products in the catalog (by category would be fine too, as we could put the "bill pay" product in its own category).  Is there a way to do this with the current Cart (we've not been ablet to gt it to behave that way)?